General - Additional upgrade slots provided by crew

If I have a crew providing additional slots for upgrades (example: B'Elanna Torres) what happens if a effect causes me to discard that crewmember (or the Borg assimilate that crewmember)?
Can I still use these upgrades?
Also, what happens if that crewmember is disabled?
Can I still use these upgrades?
Also, what happens if that crewmember is disabled?