Unclear if you understand this, so I'll just say it. You can't have 18TB data drives with a 12TB parity.
So, I'll outline what is possible with the drives you listed.
1. Replace parity with one of the new 18TB, rebuild parity and do a subsequent parity check.
2. Rebuild one of the 6TB drives with the old 12TB parity drive, do a non-correcting parity check.
3. Rebuild the other 6TB drive with the other new 18TB drive, do a non-correcting parity check.
What I've outlined keeps the array data shares and applications available the entire time, albeit at reduced performance which is unavoidable.
If at any point there are errors, those need to be resolved before moving on to the next step.