let the car get up to opperating tempreature then try to losen them, remember metal expands & cotracts so try it while the engine is hot.

if it doesnt work take it to a shop, let them break the plug off in side the head, then there responsible for the damage not u. its a fucked up idea but its better then u breaking it off and have to get a new head or have the machine shop drill out the old plug and retap the head. who knows maybe the shop will be able to get them out with out damaging anything.