As long as your services are in use, the likelihood of data corruption is high. IOW, if you still have users logged on to Exchange, that's bad. Really bad. So shutting down the Exchange services will prevent data corruption of Exchange. Since you can't really shut down Active Directory, that's the reason why AD usually gets corrupted, thus requiring a repair or restore of AD after the conversion. Thus the recommendation to DCPROMO.
And it doesn't matter that your Exchange is on your DC to do the DCPROMO thing. What that means is you create a new server, join it to the domain as a member server, then run DCPROMO on the new server to make it an additional DC. (So now you have two DCs.) Then move over the FMSO roles and eventually DCPROMO the old server to demote it from a DC. This is the same as if you were replacing your server with another physical server. But now the new server is a virtual machine, not a physical machine. Yes, it's a little more complicated, but the only way to prevent data corruption of AD.
From my experience, if you do not kick off your users (e.g.: shut down Exchange,) you're likely to get data corruption. You really must have everyone logged off in order to have even a small chance for this to work.