When I started to analyze (summer 2015) the possibilities to migrate non-ATP stores to the ATP model the IBM Knowledge Center was claiming the migration was not supported. However, not supported does not mean not possible.
So, I dig it and found a "simple" way to accomplish the migration.
Now (since 2016), finally, the switch of an existing store to the ATP mode is supported again (old version of WCS had even a migration script).
In this brief article I'll report few tips about the "migration" of existing stores to the ATP inventory model.