Use an action of the type "Replace with regular expression" for ARTIST
Search string: (.*), (.*)
Replace string: $1 & $2
Due to the greediness of the expression, only the last comma is replaced. Should work with any Artist that has comma in the name.
Will produce problems for artists that already have an ampersand in the name like "Emerson, Lake & Palmer", "Earth, Wind & Fire", "Bell, Book & Candle".