Thats not true - of course you can migrate from ds3615xs to ds3617xs with all settings.
I did this on my machine (HP ML10 gen9).
Of course it wasn't that easy - my first attempt fail.
Here's my story:
#1 attempt --> with all disks installed, boot server with new loader with install option and go to xpenology_ip:5000. Synology assistant detects migration from 3615 to 3617 and propose migration with or without settings. Of course I've selected migration with settings. But someting was wrong because whole DSM installation process was very short, less than 2 minutes, but without errors. After reboot no joy - bricked DSM.
#2 attempts - same scenario, but:
- disconnected all drives exept my first hdd (seperate 1 drive volume1 for less sensitive data)
- reflashed loader and recreated pid/vid/serial/mac
- after boot loader with install option selected, synology assistant still/again ofered migration from 3615 to 3617.
And this time upgrade worked fine, took about 10 minutes.
After succesfull 6.1.1 upgrade I've applied DSM 6.1.1-15101 Update 2 patch without aby problems.
Now: power off, reconnect rest of my drives and boot.
Used mdadm command to add connected drives to md0 array.
Everyone must remember that this migration is (I think) very hardware specific - with no proper drivers in loader it will fail. My rig is very familiar with oryginal DS3617xs hardware (same CPU architecture).
By the way, I wanted to report a strange problem:
After migration my DSM dispalying strange IP address:
Of course, DSM real IP is normal. Anyone have any clue whats couse this?