Apologies if this (or any of my other suggestions elsewhere) has already been covered.As of 1.6.1, you can't edit custom fields - you have to delete and reenter them. Would be really nice to allow this, thanks :-)
Yes you can store new custom fields using Advanced Auto-Populate, but you can't edit existing ones. Like I said, if you want to change one, you have to delete it and re-enter from scratch.
OK. Tanstaafl, can you add this to the feature request list?