Fixing RAID problem

Our hard drive capacity is down to nothing (almost). We are using RAID 5 on a Novell Server. The hard drive capacity is 4GB each, we recently bought two 18GB drives.

Can I remove a primary hard disk thus initiating a failure and allowing the network to work off the mirror drive. Then install the larger hard drive and allow a ‘recovery’ without losing overall capacity or performance.
DESPARATE
If not anyone got a solution please, pity please…

Thanks for your interest in advance…