The 3BSD licence requires (in clause 1) that your copyright notices be preserved, and that some licence text be reproduced, whenever your code is copied. But it does not require that that licence text actually apply to the copy. You can see the sort of language that would require that, in copyright licences such as GPLv3 s5c; 3BSD doesn't have any such language. I have written about relicensing at more length elsewhere, but when looked at from a list-of-conditions-on-the-copyright-grant standpoint, MIT and 3BSD are pretty much identical, so the change here is nearly zero.
I am still not comfortable having my work copied without any of my copyright notices intact
You are right to object to this. The people who've decided to remove your copyright notices are simply wrong to do so, your "collaborator credit" notwithstanding. You intimate that your work might have been published on github, which complicates matters very slightly, but if people have forked your 3BSD-licenced code, removed your copyright notices, and are distributing the result to all and sundry, they are violating your copyright. The usual remedy is a cease-and-desist letter, for which you will need to instruct a lawyer. As you are the rightsholder, the possibility of a DMCA take down notice to github applies, but that will still work better (if it works at all) when you've got a lawyer involved.
and with a different license
Since you didn't release your code under a licence that forbade that, and since the licence they've used is pretty much identical to the one you chose, I don't see this as a problem.
The project lists me as a "collaborator", when I do not endorse this new library.
In moral rights jurisdictions, this is generally wrong, and understood to be so. In other jurisdictions, I am a lot less clear that it is an issue; if you could show clear personal damage as a result of this "credit" a libel action might be a possibility, but it would be very expensive and uncertain. I would focus on the copyright notice issue, which seems to me a very clear-cut wrong.