It would be smaller steps, so easier to confirm things are working. If you're going to do this, I would install R81.20 on the new systems and downgrade them to R80.40. The R81.20 installer fixes a bug which resulted in misaligned partition layouts on all earlier versions. This greatly impacts storage I/O performance, most noticeable with small file operations.
This doesn't affect the MVC limitation. At some point, you will have a cluster with one member at R80.40 and one member at R81.10. When you fail over, you hit all the same MVC limitations regardless of the hardware situation. That said, I don't see anything about dropping site-to-site VPN connections unless they're traditional mode. Remote access connections drop.