How to change Order Number Prefix in Magento2
This is actually a standard feature since magento v1.x
But for some reason , magento team didn't open it for community version in v2.x
After digging into the codes , I found out it's actually controlled by a module named "SalesSequence".
All setting are stored in a database table named "sales_sequence_profile"
Table field "prefix" is what you want to change.