delericho
Legend
No, it doesn't. If anything about the new form makes it unable to wear those specific clothes (whether it's size changes or shape changes or, as in Elf Witch's example, extra limbs) then those clothes meld into your new form.
IMC, if the new form is one that would ordinarily wear clothes, then the character's own clothes will always adjust to match. But that's purely a matter of convenience - I don't want to have to deal with the suddenly-naked character (and the 'hilarity' that would no doubt ensue).
Besides, that seems to fit with the "fairy-tale logic" that goes with magic.
(I don't, however, claim that this is a RAW interpretation - it's just what I do.)
No, if the character changes size or shape the clothes don't change to match - and if the difference is great enough to make them unwearable, they meld.
It is worth noting, however, that if a character has magic weapons and/or armour, these do resize to fit his new form (at least in 3.5e) - that's a property of the magic item, not of the spell. But that only applies if it's just a size change that's required, not a shape change as well - so full plate +1 would resize to fit an ogre but not a centaur.
The key difference being that the game doesn't distinguish between an ogre, a troll, or a human who has received enlarge person - they're all Large bipeds and can all wear a Large suit of full plate armour, but it does distinguish between an ogre and a centaur - armour crafted for a quadruped costs twice as much. (Of course, per RAW a human could kill a harpy and immediately loot and wear whatever armour she had, without need for modification. So, I guess there's a question about how pedantic the DM wants to be about this - if he's going to allow this, should he not also allow the human, if he then shapechanged into a harpy, to retain the use of the armour in the new form? Conversely, if he's not going to allow it in the case of shapechange, surely he similarly shouldn't allow characters to immediately use looted armour?)