On 3.0 I had two custom fields set, Source (on custom 4) and trackyear (on custom 3).
Now I've upgraded to 3.1, Source has stayed the same, but trackyear has disappeared and its tags automatically moved to the new "original year" tag. It seems to have done it properly, the values seem to be correct, but there's a minor irritation.
Original year isn't an option in the drop down menu on the left main panel, meaning I can no longer use it to easily see tracks from a particular year.
It's not a major issue, more a trivial annoyance, but it would be good if it could appear there in a later update.