… where she filmed what was to become her final performance …
OR
… where she filmed what would become her final performance …
(I like the first one, as it seems to (accurately) imply that it was fully unknown that it would become her final performance… )
Best Answer
I doubt there is much difference, but to me:
talks about a definite occurrence in the past, while
also talks about the past, but from a reported time just before her final performance.