I do not think there is an easy way to convert a non-CDB database to a CDB database and here is why generally.
I believe Oracle’s logic is as follows:
There are lots of reasons to convert a CDB database to a PDB (pluggable database): 1.) increase functionality, add new bugs, increase costs (yes I’m being sarcastic).
But there are practically no reasons to take an existing NON-CDB and convert it to a CDB – as the whole point of a CDB is to store application data in a PDB…
Notice this Oracle support document goes a long way towards telling you have to convert a NON-CDB to a PDB….
See metalink / Oracle Support Doc Id: 2012448.1
Point being – if you create a 12.1.0.x database as a non-CDB database – your stuck – or at least you would have to jump thru a lot of hoops that are not well documented – including re-creating the control file… and running scripts to seed the CDB and PDB$SEED… not worth the time or effort… just drop the db and start over – unless it’s mission critical – then… let me know how you did it… thx