I’ve been enjoying my Android experience so far but haven’t done much tinkering with it. I got an Orange San Francisco and followed the instructions on http://sanfranciscoandroid.co.uk/ to unbrand and put Android 2.2 ‘Japanese Jellyfish’ on it. I see they now recommend a different ROM ‘Swedish Spring’. The only gripe I have with the phone is that sometimes it doesn’t reconnect with the WiFi when it wakes up from stand-by. I think I’ve read somewhere that that’s a ‘Japanese Jellyfish’ issue.

If I change ROM (to Swedish Spring for example) what changes would I expect to see?
Importantly, what would happen to Apps, music, data etc if I change ROM?

Depends.

Best case scenario: ROM updates seamlessly on top of old one, life continues as normal.

What may happen: ROM wipes phone during install. Music will be fine; otherwise equivalent to factory reset. Re-enter your Google Account details and all apps will be restored from the Market, although with default settings.

Worst case scenario: Something goes wrong during the flash process and the phone turns into a brick. Unlikely, and almost certainly fixable, but a possibility.

I reckon option (1) is the most likely.