You have two switches and you’re thinking that you’ll use the GBIC from your old switch on you new switch. You have an Xsan and you have a bunch of GBICs laying around and you want to know if they’ll work. You have a fiber run and you want to use a transceiver. Etc. This is a tricky question. The GBICs should all work. The general rule of thumb though is, if you use the same GBIC on both ends then you shouldn’t have a problem. But, it’s also important that (for whatever reason) some manufacturers do require certain GBICs either to actually interface or just to support an interface.