I wouldn't really be able to, sorry. I think you'll just have to test it! I mean, there's three possible outcomes:
- It changes and becomes the new Everstone nature
- It does not change because it was not actually an Everstone frame, just a frame that was naturally Bold (unlikely but possible)
- It changes the whole content of the seed; using an Everstone has a cascading effect on the way the seed processes, just like having a Ditto Parent does. But I don't know if using a specific Everstone will have a different cascading effect on the seed than any other.
I suggest you try it out and see what happens!