I cannot speak to other states requirements. I did though look into something similar here (in Texas) a long time ago so my memory may be fuzzy.

Yes you can add or take off anyone from a trust, but it depends entirely on how that trust was originally written. Here trusts are not filed with any government entity. Pretty much what the end result would be is to avoid probate or legal fights among the family etc. Anything in the trust, at least in TX is actually quite simple.

I am only speaking about in Texas, but say there was some sort of dispute, the other parties on the trust can show the documents to the judge and it's pretty much done. It's not a 'hearing' type of thing, but rather you can submit it to their office and the judge will read over it and say ok and if needed give relevant documents to those involved. Coming from out of state though, I have no idea. From what I hear, in some states trusts have to be filed in the courthouse in some cases which is totally different than here.

I would think for the SBRs the others on the trust would simply file for a permanent change of address with the ATF the exact same way as if you yourself or anyone else with an SBR would should they move to another state. I've never done it but I am pretty sure that's all that would need to be done.

The stuff not in the trust is where you would probably focus your efforts in my opinion. But like the others said, much more qualified advice is probably wise.