Nevertheless, I’d still like to know if there will be serious issues that would occur if i swapped the main HDD with another of different OS (vista vs xp). [/quote]Anything evil can happen, worst case would be a BSOD with destruction of the file system.
If that were a single HDD, I would have suggested to install the HDD additionally and then copy your files over to the new system.
Not sure if this is also applicable to a HDD that was part of a Raid 1.
Maybe you should use a live Linux CD to do the copying job since with that you can avoid any write access to the formerly raided HDD.