How would it be done, I am just asking out of curiosity.... would it perhaps involve a dynamo? or is there some sortof complex electronic stuff involved?

Ideally with a motor-generater system. Less ideally with double conversion (electronic), but they dont put out a sine wave, except the best of the best units

Today theeasiest (mainlywith respect of the big unknown what anyone connect zhere as a load) is really an electronic true sinewave type. It is definitely way cheaper (to acquire and mainly install) compare to any mot/gen unit and the true sinewave is at the end even simpler and cheaper circuit compare to any attempt making the step wise MSW to somehow tolerate and be tolerated by the various loads in use.

Most electronic units put out some sort of MSW. Some units - mostly VFDs - put out stuff as bad as unfiltered PWM of the would be sine, counting only on the motor itself as the LPF....

Changing frequencies would have to involve changing the energy from one form to another and back. It could be changed to mechanical energy such as in rotary converters or converted to DC and then inverted back into AC

Back in October, I posted an article about how Southern California's electricity grid was once 50 hz . When Los Angles Bureau of Power and Light (Now LA Department of Water and Power or LADWP) converted to 60 hz in 1936 in order to accept power from Hoover Dam. Large (185 ton) rotary frequency changers (basically a pair of motor-generator sets mechanically coupled together with one side wound for one frequency and another wound for the other) were installed at a substation in Wilmington to allow power to be shared with Southern California Edison's grid which remained operating at 50 hz until 1948.

Japan is the only major nation where both frequencies are utilized with the Eastern half of the country (Tokyo, Yokohama) running on 50 hz and the Western part (Osaka, Kyoto) running at 60. Sharing power between the two grids involved three different HVDC back to back frequency converter stations where AC at one frequency is CONVERTED to DC then INVERTED back to AC at the other frequency. This was also a major factor after the 2011 Tohoku Earthquake which resulted in the Fukushima disaster and the lost of about 9.7 Gigawatts of power capacity for that region; Western Japan had excess power available but the three converter stations could only transfer about 1 GW in total between the two grids.

If you do decide you want to try out or experiment with 50 hz , the easiest way is to acquire a Samlex or Cotek SK series pure sine wave inverter (12 volt version preferably so it could operated from a car battery) Those inverter have a switch which could be used to change the output frequency between 50 or 60 hz (and on the larger versions, you could also select 100, 110, 115, or 120 for the output voltage)

Most incandescent light bulbs probably won't care what frequency they operate on but things like synchronous or induction motors, transformers, clocks (including digital) that utilize line frequency for timing, and some CRT Televisions would be affected

Small rotary converters are not super efficient and waste about 30 % of the power when you look at input to output power, also when you have moving parts they are prone to failure, and the AC they produce is no where near as clean of a sine wave as that produced by solid state. A well designed solid state unit will actually be cleaner then the utility power, and more efficient a good design will reduce this to only a few percent, and they are way quieter, and with modern switched mode designs they will also allow for a much greater input voltage range..

Foundries where did you post that article would like to read it. A while ago someone lent me some electrical engineering magazines from the 1920s and they said that parts of southern California were on 50cycles mains I was thinking how easy it would of made things if USA opted for 220 volts at 50 c/s  I think its a bit late to change now

how hard would it be to start phasing in 220V 50Hz power to the US?

240/120 volt is already in American homes. So 240 volt
Not really a problem.  What problem is 60 hertz makes
Good time base for electric clocks. It mesns our clock will run slower. Electric induction motors will turn slower
And posssble transformer cores for 60 Hz satuates
Overheating the transformers and magnetic ballastes
Meaning it creates a whole skew of mess diue to conversions.  It just far more economical to leavie it
Alone at 60 Hz.  But modern electronics would not
Care if it 50 hz or 60 hz

Now 400 hz is another story it due to aircraft weight reduction.  Meaning they can use smaller iron cores
And less copper too.  Those transformetr u coukd never put them on 60 Hz cause it satuarate cores and windings

how hard would it be to start phasing in 220V 50Hz power to the US?

At this point in time, it won't happen, the standards are set and the cost to adopt what might seem like a small change would be in the billions, if not higher, the grid is massive and it's built to run at 60 Hz (Ok it's really between 59.95 and 60.05 Hz to allow of timing corrections) so if you need 50Hz it's locally generated or switched on site..

Indeed.  I  have been inside ERCOT it State of Texas
Electric grid controller.  They do keep it in range
They watch make sure Transmission voltage of
347 KV and current of Transmission lines
But most more in the Hz.  And it very tighttly
Controlled and power plants are Sinschronize
With in  59.95 to 60.05.  Any time  it lags
It means it loaded down to all operating
Power plants.  If demands exceeds the
Power plants it will all stall at once
So to manage the power in the grid
They watch Hz.

Now I have to manage this at home  being a mini
Grid operatot ( being a off gridder)  i have keep
My eye on voltage and Hz of my inverter as lower
Voltage will make my inverter drift off in Hz

One time in summer i put very heavy load on system
Two solar panels putting out 420 watts of power
 
Then i had a gasoline engine powering a alternator

Then including my self generating power Peddling the altetnator on a bike.  Soon as i put Heavy load on system sbout 100 amp DC load to My inverter.  Everything  came to a scrreching halt Killing gasoline engine and my self with foot sliping Of bike peddle.  With in couple of mins as i watch My battery banks  drop below 11 volt to 10.8 voltWhen my inverter kicks out.  What i sm trying say Is learning from ERCOT and Experment with My generated power.  It is extreamly hard To get power back up and running for power
Plants from stalled (black Out condition) As they have start a diesel generator Get one power plant going. Using that plant Start next one too keep it all on same 60 Hz One by one. As they would have start powering
Cities slowly until all power plants comes back Online.  

where did you post that article would like to read it. A while ago someone lent me some electrical engineering magazines from the 1920s and they said that parts of southern California were on 50cycles mains I was thinking how easy it would of made things if USA opted for 220 volts at 50 c/s  I think its a bit late to change now

http://www.lighting-gallery.net/index.php?topic=5907.0

I mainly based my post on these articles on the net:

https://gizmodo.com/before-1948-las-power-grid-was-incompatible-with-the-r-1683629042

http://waterandpower.org/museum/Mystery_History_Answers(April_2015).html