I have a Go 550 that is almost a year old. So far I haven't patched it as I didn't want to risk upsetting the live services, but at some point I willl want to update the map.

Is it safe to assume there will be no affect to the live service subscription if I patch the device using FastActivate and update to a new map? Will I be able to renew my subscription through TomTom for Live Services without issue?

As an aside: If I sold the device, what happens to any remaining Live Service subscription? I would want to reset the device before posting it, so would this remove the subscription or is it linked to the device itself rather than my TomTom account?

Live services you paid for can be transfered by TT to a new device if you ask them.
A new map won't affect your services either.