merging access filters?

A quick search of the help didn't answer my question, so I figured I'd ask my readers before asking DW support (sometime after Pesach, which starts tonight).

We can merge tags, i.e. delete one tag and send all its posts to another tag. I want to do the same thing with a couple access filters; I have more granularity than I need. If I just delete an access filter all its posts become private; that's not what I want. I want to take this group of posts that all have access filter X and assign them to access filter Y instead, so that I can then delete X with no negative effects.

Anybody know of a way to do that?