The same way it's done with Filename-Tag conversion.
Let's say we have a song with the title "01 - Song", and we want to change it to "Song".
We select Convert -> Tag-Tag
Program asks for field and we select: TITLE
Program asks for formatstring and we enter: %1 - %2
(Program shows preview - 1: 01 and 2: Song)
Program asks for new entry and we enter: %2
Press OK.
About 42 times simpler than using your suggested method.
