I have found many instances of gifts that were allocated to the wrong recurring gift on some constituents who have more than one recurring gift. Adjusting or even deleting and re-inputting the gift is more work than is needed if there was an un-link option.
This seems to be an ongoing issue. Hopefully BB will add the ability to change which anchor is linked to the payment after GL Post Status has changed to Posted. I tried to Unpost with a BB Rep and then change the anchor, and it still didn't work.
See also https://re7.ideas.aha.io/ideas/RE7-I-339