Hi everyone, I found an old Fidenza Vetraria Polaris fixture with a white light lamp inside, I imagine it's mercury vapor. Can I replace it with an HPS lamp or do I have to change something inside?

There are high pressure sodium retrofit lamps specifically designed to be used in mercury vapor fixtures. However, there is one major limitation to consider whenever it comes to directly replacing a mercury vapor lamp with a high pressure sodium lamp:

Most high pressure sodium retrofit lamps are only compatible with HX autotransformer ballasts and series choke ballasts and they must NEVER be used with CWA ballasts.

If you have a CWA ballast, the only compatible high pressure sodium retrofit lamps are Philips North America’s Retrolux lamps (some branded “CEW”), Iwasaki’s Sunlux Super Ace (LC) lamps, and Iwasaki’s Sunlux Ultra Ace (CE) lamps.

If you live in Europe, you should hardly have any problems with using high pressure sodium retrofit lamps on your mercury vapor ballasts since most of them are simple series choke ballasts. Sadly, they are pretty hard to find in Europe since they were banned in 2015 along with mercury vapor lamps. I think the only high pressure sodium retrofit lamps still in production are found in Japan and they include Iwasaki’s NH360LS lamps and Panasonic’s NH360LN lamps, but those use E39 bases instead of E40 bases.

If you are using a standard high pressure sodium lamp, they require different ballasts than mercury vapor lamps of the same wattage and they usually require an external ignitor.