Can changing iPad motherboard to older version bypass iCloud lock?
For most intents and purposes, the motherboard is the iPad. The rest of it is just parts which surround the motherboard -- case, battery, screen, etc.

All of the device's storage is on the motherboard -- placing a new motherboard into the shell of another iPad will not give you access to data which was stored on the old iPad, because the storage was all on the motherboard you removed.

If your goal is simply to have a working device, you can continue, of course. But if you want to get access to the data you had on the old iPad, you'll want to look into other solutions.