Comment by mandevil
If you have done the out-of-band safety number verification, then impersonation attempts will give you a warning that their safety number has changed. I know this because I got that error when my wife replaced her cell phone.
I believe (though I haven't verified it myself) that even if you haven't verified the numbers using an out-of-band exchange mechanism, you will get a warning if the safety number as observed by their server changes. I believe they would need to know your Signal PIN to restore from backup, which means that even if you've set that it will give an alert, presuming basic security competence from the people you are conversing with.
> If you have done the out-of-band safety number verification
I personally have never seen anyone do this, even when they’re supposed to do it right from the very beginning. So practically this is of very little value to most of the user base.