FreePBX 12 migrating from 2.11

Is it possible to migrate the configuration from a physical FreePBX server running 11.9.0 to the latest FreePBX 12.0.x?
I tried to restore the complete backup but seems to corrupt the database.
We did some tweaks in the extention part (extention.custom.conf) and we like to take that over. The file itself is not the problem but the PBX settings are.
What am I doing wrong or is it not possible at all?

Thanks in advance,