How much a restoration affects value depends on how well it's done, and the buyer.
Some collectors will not touch restored ponies, aka rehair, repaint or reglitters. Others will but only as a placeholder until they get a better more original version, and still more may not care as long as the pony looks original. A poor job where the pony is notably different (inaccurate application, bigger glitter) almost always will reduce its value. Personally I would not be interested in a pony with reglittered symbols because I've yet to see anyone do it perfectly enough to match factory issue and due to the conservation concerns already raised.