When your license expires your indexes will keep indexing but searching via search heads will stop working.
If this is a critical issue speak to your sales rep regarding this and see if you can get it reissued with a week/2 weeks inbetween.
You may be able to trick the license server into accepting the non valid license by changing its time while splunk is stopped but you'd have to stop anything from calling in while doing this (shutdown network interface or something). Start splunk, install the license (valid as your machine is in the future) then shutdown again. Correct the time and then restart splunk and i'd expect the license wouldn't be active (but still listed as installed).