Is there software that can write the AT&T IMSI to my own sim and leave the KI alone? Then I wouldn't need a silvercard and I wouldnt need to brute force the KI.

Also I'll keep a note of the original IMSI and write it back if I need to.

Is this possible?

I had this idea too but there is no way to do it buy yourselve BUT!

Maybe your Provider will be so nice and just change your IMSI to the IMSI from you AT&T Card! You will need a new Simcard from you Provider but where is it writen down that a Custemer cant ask his Provider for a spezial IMSI