You can use a case conversion action with the field specified as _FIELDNAME. This will rewrite all field names with the designated case conversion. What I recommend is creating an action group to do this, and making sure that it is the last action group that is executed whenever you manipulate any tags. You can also run it by itself using the quick actions button.
You should also find Mp3tag's file view (columns) definition file, named columns.ini and make sure that all of the field names are written in upper case. There will be many column definitions in the file looking something like:
[#18]
width=246
numeric=0
name=Title
value=%title%
sort=
field=%title%
enabled=1
Make sure all of the 'field' entries are all upper case. For example:
field=%TITLE%
Also, any custom fields that you've created in the tag panel, which are defined in usrfields.ini. Make sure all of the 'value' names are all upper case.
[#2]
value=ALBUMARTISTSORT
name=Album Artist Sort:
multiline=0