CCE Cloud PBX deployment

Since there is already an invested and established Hybrid setup in place in production environment, I woudn't suggest you flatten all up instead make the best use of it by moving Online users back to Hybrid -- primarily because they are, I assumed "Pure Online users without Enterprise Voice" -- minimal migration time, and less administrative efforts are your advantages to name a few.  

After all, CCE is a minial and stripped down version of SFB running on-prem that you still have to maintain it -- upgrading to a newer CCE version, configuring with new features like Media Bypass, etc and of course hardware. 

At the same time, the licenses you subscribed for current Online users can also be used/assigned after you enabled them to be Hybrid users.