no just leave the old elastic there hon....sew over the top of it. It is really easy once you do one or 2. you'll find most big brand multi manufactored nappies have elastic that has been sewn on the ends...its perfectly normal and there's lost of nappies out there done like that. thats why they have elastic casings as such IFYKWIM
Ive fixed so many IB's, baby beehinds and bum G's this way and it really is simple and quick...go for it I say...YOU CAN DO IT!!!!