R520 - interchangeable system boards?

The 51XDX board is a 2nd release version of the systemboard, where the 3P5P3 is a 4th release version of the board. The things you will need to consider is with replacing the system board, you will replace the iDRAC as well, as it is embedded in the system board. So you may need to do the following;

1. Re-licensing may be needed. You will need to supply the license key, which should be imported after system board replacement.
2. iDRAC reconfiguration may be needed. You will supply the configuration file, which should be imported after system board replacement.

Once those steps are done then I would verify that the server is up to date on updates as well.

Let me know how it goes.