Tvelchek,  The issues you will run into is with OS stability, as the System State information from one server won't match the other. Transferring drives over containing data doesn't cause issues, but it is generally more unstable with the OS array. Also there is new hardware in the R420 that wasn't preset in the 410. So you will run into issues with hardware drivers as well.  Let me know if this helps.
I've done this with many Compaq servers. I would like to know if there is any reason why I couldn't take the drives (boot & data) out of the R410 and install them into the R420. I've looked and can't find much difference between the hardware on these machines, so I'm expecting little confusion\complaints from Windows SBServer 2008. Any experience? (or thoughts?) Troy 
HDD swap from R410 to R420 rack chassis.