It is not possible to overwrite or re-define an existing tag-field from the Tag-Panel's standard set of tag-fields.
You can create an additional multiline Tag-Field="MY_COMMENT" with Name="My co&mment".
To avoid the appearing of the new defined tag-field in the list of tag-fields, ...
you may set a Mapping for Tag=ID3v2 with Source=MY_COMMENT and Target=COMMENT.
But there is a problem when subsequently writing text into the MY_COMMENT edit field and saving ...
because Mp3tag adds multi-value COMMENT tag-fields.
DD.20130118.2235.CET