For anyone thats interested.
The migration from 54L to Gen10 was straight forward.
1. Create new boot image on new USB (Jun 1.03b - DS3617xs)
2. Transfer drives from 54L to Gen10 (same order as in 54L)
3. Insert boot USB into motherboard
4. Change BIOS to boot from USB and "UEFI with CSM module" = DISABLED and "Boot Mode" = Legacy only
5. Boot Gen10 and use Synology Assistant to find NAS
6. Was prompted to Migrate NAS, I used PAT file DSM_DS3617xs_23739.pat
7. Create admin account and set-up Gen10 as needed
Not too concerned that I'm not running very latest DSM, more relieved the upgrade worked without a hitch.