I started the same with N54L full of data and DSM 6.1.7-15284 (Jun's loader v1.02b - DS3615xs) and i've read a lot about upgrading to 6.2.2. I know then it will not work with onboard NIC. But now DSM 6.2.3 is out and it is not necessary to change the NIC. The onboard one will work fine without external drivers.
So i made a new USB Stick with Juns v1.03b - DS 3615xs, booted and found the NAS migratable in the Assistant. I selected to keep my files and installed the newest 6.2.3.pat (DSM_DS3615xs_25423.pat). I made a reboot and waited, aber 10 min i thought i bricked it, nothing happened. BUT THEN after 13 - 15 Minutes, it showed up in Assistant with a new IP (DCHP) and Status: finished!
Everything worked fine! In the DSM a couple of apps need to repair but now all data is there and everything worked finde.
Thanks to the forum!