No worries... I was pretty sure that's what you were going for with the FOS version.  Those steps are pretty much what I would do (and have done) in the same situation. Just make sure you review the troubleshooting guide for all the things that could prevent the switches merging into a fabric. That's what will usually catch you up. Since it is a new switch with nothing on it yet though there is no real risk. If you try and ISL and it doesn't take you just figure out why, fix it, and try again.  As for pulling the ISL down when you are done... yes, you will end up with the same zoneset on both switches and you can proceed with decommitting the old one. The new one will now act as a stand alone fabric and continue working as it was when the ISL was in place.
So we have 1 switch fabric [4100B v.4.2a] that needs to be decomed after the cables being migrated to 6510[FOS 7.2.1].   Will ISLing the 2 switches segment the switches?  I wish to (after the ISL's) move the cables from old to the new switches and remove the ISL. Will this make the new switch principal with the inhereted zone(& zoneset) information from the old switch?   Please let me know. 
ISL between 2 Brocade switches with different FOS versions Options