In my mind, they should be paying the actor the same for the new lines regardless of whether they opt for them to come back in and re-record or use AI to generate the new line. The actor’s product (their voice) isn’t worth any less, but the company could save money by streamlining the creation of a new line through simplified logistics. This way the company has some benefit while preserving the actor’s livelihood.
Of course there’s no way these companies are going to want to pay full price for these new lines, since it’s an obvious point where they can pressure performers to accept a lower rate.