Similar, yes. Same, no. The PC's 256K board was made such that it was a real hack-job to upgrade it to 640K. The PC/XT upgrade was trivial, as described here. The motherboards were *not* the same.
The PC-Jr was the same. The 128KB side-cars were easy to upgrade to
512K (suck out the 64K ships and replace with 256K - add a jumper, IIRC), for a total of 640K.