Depending on how old your ac evap core is, you might think about replacing that at the same time. If you plan on removing the dash completly, I would change the evap as well. It is only a few steps more, but it will save you a TON of time if it starts leaking in the near future. It's just one of those things where replacing the heater core, you are 3/4 the way to replacing the evap.