Unfortunately your procedure won't work unless you get ANOTHER 12TB drive. No data drive can be larger than either parity drive, so until both parity1 and parity2 are 12TB, you can't install a 12TB data drive.
The parity drive holds zero data, it only works in conjunction with all the other data drives, so as long as all your data drives are healthy, and a very recent parity check had zero errors, swapping the parity for a larger one is a infinitesimally small risk. It's pretty much a zero risk if you do it in maintenance mode, since if a data drive died while you were upgrading parity you could just put the old parity back in.
Also, since you are scared of data loss, I take it you have no backups? Parity isn't a backup, its only purpose is to allow a single dead drive to be rebuilt. Deletion or corruption of your data, from any source, isn't recoverable with parity, you must have backups.
I think before you start upgrading your server, you need to get your data backed up so that if something does go wrong it's not catastrophic for you.