Although all of your questions are quickly answered when searching this site and/or Googling around for a few minutes:

When an I/O interrupt occurs, the processor stores the address of the next instruction on the stack and jumps to the address vectored to by the interrupt. These addresses are fixed for the 8052. The jump made to the address is a subroutine jump and a RETI instruction ends it, causing the stacked address to be retrieved and execution to continu at the location following the interrupted instruction. And no, the addresses of the ISR's can not be changed, but you can (and normally should) provide a jump table from there.