The upgrade is likely to not cost anything, it will probably be a "Petrel Firmware Update", and all petrels will get it. If it has the compass and advanced bluetooth chip in it, then it will work. If it doesn't, it will be just like it was. So I don't think this is an issue. They made it very clear, if you have hardware version 2.4 then when the firmware comes out, it will upgrade itself to the Petrel 2 and just activate the new module. Not sure what else there is concern about?
All of the shops have the Petrel 2 at the same price as the original Petrel. Mine is only 3 months old, I wish I had waited just to get the next model, but since everything else about it is the same, I'm not worried. I would also probably never use the compass, and since I don't have an IOS device, I'd never use that either.