I've always wondered this, but where do municipalities, or whoever owns the streetlights put the old HID streetlights when they upgrade to LED?

Usually scrap or recycling yards.

They usually get scrapped or recycled.

However some might end up getting resold in auction as surplus on sites like GovDeals or saved for spot replacing other HID luminaires if the municipality still has other HID in use. Sometimes older replaced lights get reused as temporary lights as well.

Sad they just get scraped or recycled. 

They've got a huge (7m IIRC) E-waste skip full of old streetlights here. Oh, how I wish I could go for a dig! They then get disassembled and have each component properly recycled.

That would be super fun to dig through, imagine how many streetlights you would have!