The other thing to know is that the Resound receiver can be used with a mould or a dome. There is no difference. You need to have the right power (MP, HP or whatever) and the right length of wire. Size 3 is probably a good starting point if you can’t see what is written on the existing one.
It can be hard to get the receiver out of the existing mould without breaking it, so starting with a new receiver is a good plan.
You should really change the settings in the fitting software after changing the mould to a dome, but if it works without feedback then don’t worry about it.