If your LBS put Loctite on the threads you may need to put some heat to the crank where the pedal attachs. Heat will help melt the Loctite and make it easier to loosen the pedals, I know this because I work in the automotive industry and we do this all the time to loosen Loctite. You can try a hair dryer and concentrate the heat to the area where the threads are. When you install the new pedals grease works OK, but antiseize works much better. Antiseize stops the galvanic action between the dissimilar metals, grease will not and can washout leaving you with no protection. Do not use antiseize as a lubricant, it isn't made for that and doesn't work worth a darn. Good luck!