I think for any kind of hi-def treatment they are going to need to source from the original film negatives. It doesn't seem like editing the original film elements back together should be all that difficult. It's the fact that they'd have to replace every single transporter and phaser effect that would make this really time consuming. TOS at least could use the existing transporter and phaser effects and save their budget for the background and space exteriors. All this stuff on TNG was done on video tape.