use 1001\customer again in 0 alias oldcustomer shared 
use 1002\customer again in 0 alias newcustomer shared
sele newcustomer
set order to accode
sele oldcustomer
set order to accode
go top
do while not eof('oldcustomer')
     scatter memvar
     sele newcustomer
     if  not seek(m.accode,'newcustomer','accode')
         appe blank in newcustomer
         gather memvar
    else
          if empt(newcustomer.address1)
             repl newcustomer.address1 with m.address1 in newcustomer
         endif
          if empt(newcustomer.address2)
             repl newcustomer.address2 with m.address2 in newcustomer
         endif
    endif
    sele oldcustomer
    skip
enddo