Mp3tag is not able to detect a group-change resp. cannot compare tag-field content between two adjacent files (in the interactive part of Mp3tag).
Such comparing would be possible only within an Export script, which analyses the given tag-fields and creates output to text file, from which in turn values can be imported into tag-fields.
See also ... unresolved ...
I wish I could always $get() a variable
DD.20120602.1824.CEST