My procedure with a new Solo
kit has been to fire-up the new controller with the new Solo (and gimbal - and a GP) to test basic system functionality within the 30 day warranty. At that time I download and update both controller and Solo, then put away the new controller. If they are sending you an entire new Solo kit, you can do that.
I wouldn't see it as a problem with initially mismatched FW anyway - I suspect it will automatically update the 3DR device that needs updating, and perhaps automatically update the 3DR device that doesn't need updating - or not, if the update checks versions first (which it should).
I will say, that I have a habit of trying 'stuff', and asking questions later - or at least if I am relatively comfortable that what I do won't make things 10 times worse. 2 or 3 times worse, is okay!
If you don't have
any mods on the old Solo, I'd try the new Solo a few times and send the old Solo back.
However, I have understood that warranty replacement Solos are refurbished, not NIB. I cannot personally confirm that.