When we were deploying O365 both the partners and MS said "you don't have to uninstall previous versions, they can stay on the computers". To which I said "why leave them on there?" and they didn't have a great answer for me other than "you can uninstall them if you'd like".
The below wasn't around when I was deploying...it would've likely been overkill for my needs and we had multiple users that still needed Office 2010 around in conjunction with Office Pro Plus 2013 for a while...but I had bookmarked this last month for another potential client.
That said, here's what I had found.
In conjunction with the PowerShell App Deployment Kit found here:
Christian Fosund has a script on his site he's using for a very large deployment:
This is the script I have been using to deploy Microsoft Office 365
Pro Plus to nearly 5000 clients. Its based on the awsome toolkit
PowerShell App Deployment Toolkit by Sean Lillis It will uninstall all
previous versions of Office and also remove additional applications
like Live Meeting, Communicator 2007, Conferensing Add-in etc -- Christian Foslund
It's not a "download and run it" script. You'll need the PADT and you'll need to change the script code for your particular environment needs, but it appeared to be worthy of a bookmark, and maybe it will help you as well.