Amber for Lumia 620 is already available via NSU!
Amber for Lumia 620 is already available via Nokia Software Updater for Retail. I have updated my phone just a moment ago. But be aware- it restores your phone to the factory settings, all user data will be gone after update.
Probably Amber update for other Lumia phones is available via NSU as well, I am not able to check it.