if you have common XDATA and Xprogram try following

ORG 0000h
    jmp Reset
ORG 0023h
    jmp FixedLocation in XRAM
............
............
Reset:
     mov dph,#HIGH (FixedLocation)
     mov dpl,#LOW  (FixedLocation)
     mov A,#02h        
     movx @DPTR,A
     inc DPTR
     mov A,#MSB_of_add_of_ISR_in_Internal
     movx @DPTR,A
     inc DPTR 
     mov A,#LSB_of_add_of_ISR_in_Internal
     movx @DPTR,A

     ..........
     .........    ;downloader code


;and when all done
     clr EA
     mov dph,#HIGH (FixedLocation)
     mov dpl,#LOW  (FixedLocation)
     mov A,#02h        
     movx @DPTR,A
     inc DPTR
     mov A,#MSB_of_add_of_NewISR_in_XRAM
     movx @DPTR,A
     inc DPTR 
     mov A,#LSB_of_add_of_NewISR_in_XRAM
     movx @DPTR,A
     setb EA

Abhishek