The SyncWord is always configurable. It is here to separate networks and avoid spending time demodulating frame which belongs to another network, so if you are on a completely different band you can re-use a syncword without worry.
As soon as you are on a private network you can use whatever valid SyncWord you want as long as it is different from the public one. I think the value 0x12 for private network was just defined to give people one valid value which was tested and characterized. But if you set up multiple private network in the same geographic area you will need to use different sync word and if you respect some rules it will work properly.
As for the rules:
- Each nibble of the SX127x SyncWord must be strictly below 8 to be compatible with SX126x
- Each nibble of the SX127x SynchWord must be different from each other and from 0 or you might experience a slight loss of sensitivity
- Translation from SX127x to SX126x : 0xYZ -> 0xY4Z4 : if you do not set the two 4 you might lose sensitivity
There is more to it, but this should be enough to setup your networks and hopefully the official response will be more complete.