Could one of the drives be switched out for a 2TB 6Gb/s @ 7200 rpm?
Short: Yes, without problem.
Long: Most RAID levels RAID does not need synchronised spindles. (Exeption: RAID levels 3 and 4, which nobody uses. See this link for commonly used RAID levels and this link to wikipedia for RAID level 3 (byte level striping) and RAID level 4 (block level striping)
How do you know if two drives will be compatible in a RAID array?
If the replaced drive is equal in size, or larger than the drive it replaces it just works. If it is smaller then you have to do some work.
Note that 2TB drives are usually drive of slightly less than 2TB, rounded up by marketing. There is about a 50% chance that one one of your drives is smaller. Hopefully it is not the new one.