How to copy (if necessary) all ID3v1 tag values to ID3v2 tag values?

I want to get rid of the ID3v1 tags of all marked mp3 songs and keep only ID3v2 tags.
Problem: ID3v2 could currently not exist or ID3v2 tag header values could be different from ID3v1 tag header values.

How to solve this?

At first I enabled read and write of ID3v1 and ID3v2

According to the documentation changes of tag values will write them to both ID3 tag headers.
But only when they are changed.

But how do I tell Mp3tag to copy all (~ 300) ID3v1 tag header values to the corresponding ID3v2 tag headers BUT ONLY IF NECESSARY (if they are different)?
AND WITHOUT making dummy changes just to trigger the copy operation?

To copy all V1 data to V2 data:
Set Tools>Options>tags>Mpeg
to read V1 and to write V2
load the files,
select the files,
press Ctrl-S to save.
Now everything that was stored in V1 tags is written in V2 tag data format.
MP3tag does not compare the contents of different tag versions.
It would be up to you to sort out which tag data is the correct one.

If your read and write V1 and V2 then you see only the data from the V2 tags as they have priority over V1 tags in MP3tag.