The problem is that of your player which apparently interprets "semicolon blank" as a separator. (I know that WMP does.)
At least WMP displays all entries in the track list and only separates the fields for the sorted list of individuals.
But your question was: how to replace a ; with /
Create an action of the type (simple) replace for COMPOSER
Search string: ;
Replace string: /
(add the appropriate number of blanks where you need them. The forum editor does not show them properly)
Do the same for ARTIST if you want to replace the ; there as well.
Here is some more help on actions: /t/967/1