Rig UIs generated with Rigify in 2.7x won't work correctly in 2.8 and have to be updated.
This can be handled by the Rigify generate function, the whole rig would be overwrote tough.
We dealt with similar design issues moving from 0.4 to 0.5. In that case opening a blender file containing an old rigify rig/metarig would have displayed a warning giving the user the option to upgrade the rig/metarig to make it work in new version (displaying also a confirm pop-up disclaiming feature may change)
run rigify in legacy mode and ignore new features.
From an user point of view i think would be better if we could create a similar feature for 2.8 too.
So we have here 2 options in my opinion:
automatic: if rigify is enabled, and detects an old rig, re-generates the python scripts
manual: if rigify is enabled, and detects an old rig, displays the user a warning and the option to re-generate the python scripts
the interface can display a button like the one we did for legacy rigs just replacing "Upgrade metarig" string: