Bruce,
My questions: If we don't upgrade to Premium beforehand, what files
will get discarded during migration?
None.
This happened to me in transferring one of my groups. The transfer agent suspended the copy action and emailed me to ask what to do. In my case it turned out to be an excess of photos (1.7 GB) and the solution was to turn on resizing, selecting a 2000 x 2000 max was sufficient to bring them under 0.75 GB.
Does groups.io notify you when you go over your storage limitation and
provide a "grace period" for you to do something about it before they
begin to throw out your files and photos?
When you're at quota I believe you are prevented from adding more rather than having anything automatically deleted. The exception is message attachments, there's a group option to delete oldest instead of rejecting the new messages.
Shal
Shal Farley