I'm a bit confused.
What is the P64 Pro? An off-brand light?
Or do you mean the 64 LED Pro, like what I have 8 of?
If I understand MyDMX properly, if you remove a fixture, you REMOVE EVERYTHING that went with that fixture. You also can't "upgrade", you've got to add in the proper lights with the proper profile. To me, it sounds like a failure to really plan ahead.
You, like others of us, are trying to avoid the obvious route of having to do lots of work. Sad to say, you're going to need to do some work. What channels are you using? This might not be easy.
You can still save your existing show, just make a new universe for your 7-channel profile configuration.
Sad to say, you can't have it both ways. MyDMX isn't designed with that soft of functionality. Also, one thing I had to consider is that just adding a channel means nothing, because it's how that fixture works in specific channel modes, and it's not an interpretatin thing. The software is just looking at values per channel.
I doubt high end consoles would do this either.
Time to break out pencil and paper and start documenting. I wish I had another answer, but I don't. Unless you could convert the DMX information to MIDI data, save that to a sequencer, but even so, you'd still have to manipulate and interpret, and you're not going to get results easily.
Crack open some Red Bull, you're in for some long nights.