I just did the same thing you are asking about. I actually migrated from a dual 3tb shr drive setup from DS213 to hp microserver gen8 running esxi 5.5, gnoboot alpha9 and synology 4.3.
I took one of the 3tb drives out and stuck a new one, both in RDM mode so synology saw a
Degraded volume. I added another 3tb drive and let it parity check for 1.5 days. After that, I added another 3tb drive and it took 4 days to expand volume. After that, I had to click another option to expand volume but it would stop right away. In the logs, there were comments of file system errors so expansion could not occur. I read some tutorial to log into the shell and run fsck.ext4 and after a reboot, I could expand the volume.
Good luck!