give em a good wiggle and twist. most of the time its improper installation that makes it hard to remove them. worst come worse you pull the wire apart. your already replacing it so no problem there. are you going plugs as well? if the wires arnt wanting to come off nicely that usually means the plugs havent been serviced anytime recent.
they are a bit stiff to get off because of the design of the wire. force wont harm it as its being replaced. only thing that can go wrong is it wrecking the plug. which if your replacing them at the same time no problem.
if your wanting a nicer install and replacement later, get some dielectric grease, put some on the contant of the wires then pop them on. should hear a good snap as it seats on the plug. the grease will make a better contact, keep moisture out of the contact and make it easier to replace later on.
this happens alot on some cars as the plug wires get left on so long (our corolla didnt need plugs and wires changed till 192,000km, those were a bugger to get off)
