Many a times you may not be able to exhaust your data bundle and thus need to roll it over after expiring date with the new sub. Have been doing that for a while now successfully since the feature was rolled out to all subscribers a while ago.
How To Roll Over your Unused Data[corner-ad id=1]
It is very easy
<1> Subscribe to a data bundle
<2> Before expiring date of your subscription, resubscribe to the same data bundle and your left over will be rollover with the new subscription validity.
Note: The above is for MTN subscribers.