Replace " / " with ", " except when there is only one " / " or it is the last " / ", then replace with " & "

Support
4

Then try the following:
First (simple) replace all slashes / with a comma.
Then try the following "Replace with regular expression":
Search string: (.*), (.*)
Replace with: $1 & $2

The greadiness will take care that this always happens only to the last comma.