well, strictly speaking REMIXES are usually done by juvenile artistes who capitalise on the popularity of earlier songs. the original song is taken and a layer of new sounds are added. real remixes are:
chandramukhi remix (where dialogues from the film are added to the soundtrack, with a new heavy thumping beat.)
loosu pennE (the fast beat - where yuvan has just added a layer to the original song (he composed) to give it a fast tempo feel).
oh ho ho ilamai from varalaaru - the rabbit mix.
re-recording an earlier song, strictly speaking is not a remix. one can call it a remake. examples include:
adhO andha paravai pOla vaazha vEndum (Madrasi - music Iman)
engEyum, eppOdhum sandhOsham by Malgudi Shuba (private album)
ARR's thottaal poo malarum does not fit into this category, as the tune is completely new.
besides those above, if a tune of an earlier song is used for a new song, it is simply a copy.
having said all these, i feel that a MD of yuvan's calibre should stay away from remixes and remakes and leave them to lesser MDs like Vijay Anthony and D Iman. :D
note: thee pidikka and namma kaattula are neither remixes nor remakes and definitely not copies. parts of old songs (originals) have been inserted for nostalgia and other effects. i personally enjoyed both songs very much.