We basically need to run a powershell script against a group that turns that exchange license on under the E5 once the person is migrated to Office 365.

Easy way to handle this would be to create a License assignment policy in Azure AD. Set it up with a new group and add users to the group as they are migrated. Assign the E5 license fully to members of that group with the policy. Voila. 

actually let me provide some additional info on this,

We already have a script that runs for new employees which licenses say 15 of the 20 options under the E5.  Exchange is not one of those options.

Once migrated to exchange, we wouldn’t want to license the E5 fully, we would just want to add exchange under the e5 so do I set up a license group with just exchange under the e5 and it adds it or do I need to set it for the existing 15 options, plus now exchange making it 16 of the 20.

Here's the script I usually recommend for such scenarios: https://gallery.technet.microsoft.com/scriptcenter/Change-Office-365-licenses-e3a26eb0

hi Vasil, can this be used to "enable" just the "exchange" option in the E5 license feature set?

that's great just what i was looking for.  Thanks Vasil!