Will my old island, weapon and vehicle addons work with OFP: Resistance?

Most of them will but the missions that reference them may need to be modified to conform to this addon declaration rule, introduced by OFP: Resistance.

By the way, when you play missions in OFP: Resistance that take place on the original islands, you'll still see the old textures and models being used for houses and foliage but the old island's terrain geometry will be smoother than before.