What mesh would need altering?
You don't need to change the teeth meshes. Teeth textures are what really matter.
Would I need to retexture?
Yep, you'd want a texture that shows a row of pointy teeth.
How can I get the player character to change...and how do I get NPCs to change....to the sharp-toothed form?
If you edit the vanilla Wood Elf race in the CS, all the wood elves in the game will be affected including the player character.
And can such a change be gender specific if wanted?
Face data (except for ears...) is not gender specific, so no. Both sixes use the same teeth.
Here's a suggestion. http://tesnexus.com/downloads/file.php?id=16366 have http://tesnexus.com/downloads/images/16366-2-1207326117.jpg. I'd use theirs. Two things though:
1. Their gums are greenish. I'd make a recolored version for wood elves.
2. The teeth texture is meant for a merged teeth mesh, so you'd want to either split the teeth texture into 2 (upper teeth and lower teeth) to make them fit separate teeth meshes OR more simply make wood elves use merged teeth. All the vanilla races in my install use the merged teeth and mouth already, so that would be a non-issue for me.
Assign the pointy teeth textures to wood elves in the CS and you're theoritically done.
If you already have a cosmetic mod that affects wood elves, you'll want to either tag your little teeth mod with the correct tag (R.Teeth I would think) and import it in your bashed patch OR make the teeth changes in said cosmetic mod. Else the two mods will, y'know, fight to the death and all that.