i want to get my windows tinted 15%, but before i do i want to replace my scratched windows, all of them have the common lines down the middle.
I have a full set of unmarked ones to put in but they have some crappy light tint, Whats the best way to remove the old tint with out scratching the glass

That's a great question as my tinting is getting pretty crappy and needs to be replaced. You can do it yourself of course and I have done it once in the past. It came up beautifully except for one window where the wind blew the film together just as I was fitting it and I got bubbles/creases and didn't have enough film to make another piece. I never did get around to re-doing it. I would recommend you film the windows out of the car since you have a spare set and clean super spotless before applying the film. I will have a hunt for some old photo's of my TRX that I did myself. The film was only $20.

I'd just ask the guys to remove it when they put the new stuff on... couldnt be more than a few bucks.

Heat gun and a scraper works pretty well.
Do it carefully, only apply enough heat to wrinkle the filmnot enough to burn/melt it. Scrape gently to avoid scratching the glass. Clean the glass afterwards with a non-amonia based cleaner.
If you're keen to have a go yourself and think you can get the new glass in without scratching the tint, tint the glass before fitting it.

use a plastic scraper too

The tint joint only charged me $40 extra to remove the old tint.

From memory mine was around that mark too, from the tint professor.
I wouldn't bother with DIY tint, just about every tint place offers a lifetime warranty now, its worth it IMO, if it bubbles you can just take it back.
For the extra 40 odd bucks, let them worry about removing it.
If they scratch it, then they can replace the glass.