I've uploaded an action to:

https://2manyrobots.com/actions/AS-Remix.zip

I have not done exhaustive testing but seems to work.

Test 1:

old Title: test (john remix) (abc)
new: test (abc) [john Remix]

Test 2:

old Title: test (abc) (john remix)
new: test (abc) [john Remix]

Test 3:

old Title: (john remix) test (test2)
new: test (test2) [john Remix]

Test 4:
old Title: (abc) def (ghi) jkl (mno)
new: (abc) def (ghi) jkl (mno)

The action makes the changes but does not save the results. A Save statement can be added to the action if you wish.

Have fun.