If they want to use the cloud migration tool they would need to be on GP2015, however what I have been seeing is that customers are wanting to clean up their ERP. They are wanting to change their COA and take this time to reimplement.
If they use the cloud migration tool they would have to upgrade, not change anything in their COA. Also, even through they use the migration tool you still have to go through the environment and do clean up after. The detail transactions DO NOT come over into BC it summarizes by year by period.
So it is really up to the customer is quite a bit of expense to upgrade and most of the customers I am working with decide to reimplement.
Hope this helps!