Sorry to head about the problem. You have one clean looking gp there! I'd try searching under Billboost as he has a lot of how-to's like this.
It might be "worth" trying the fluid change and filter, but I wouldn't expect anything, and even if it does "work" I would save up for doing a rebuild or replace sooner rather than later.
If you have the 04 gtp non-comp g, you can't use the comp-g trans unless you have the computer flashed for it and install the shifter and paddle shifters. The valve body and channel plate are different as the gearing ratio. I can't remember, but around 04 the transmissions changed the shift selector to internal and the checkball and a few other minor things, so make sure you look around and make sure the one you're getting works since 04 is an odd year.
As far as the general process, you'll need to remove the intake to get to the upper bellhousing bolts. Remove starter (think it's two or three bolts). If you have headers, I usually find it easiest to "feel" the bolts for the bracket attaching the engine to the transmission by reaching down the back laying over the engine. I found it easiest to use a fender brace engine holder to hold everything in place, but you can use a lift or something similar. You'll be dropping it out the bottom, so unless you want to do the engine with it, a jack won't work. I usually unbolt the 10mm dipstick tube and pull out first. Remove all electrical connectors by the intake side and the differential mounted speed sensor. I usually remove the speed sensor so I don't damage it. Remove the two differential bolts attaching the engine to trans bracket. Un-clip and remove all the transmission cooler lines. Unclip any electrical connections and basically anything attached to the subframe. unbolt the trans and engine mounts. Unbolt the power steering rack from the subframe and watch out for any eletrical connections. Unbolt the sway bar. Remove the torque converter bolts by spinning the flywheel with a screwdriver or something and removing bolts one by one (there's three). I usually drop the pan and let the fluid drain out (it's lighter and easier to work with) then use a few screws to re-attach. Remove your wheels with the vehicle jacked up and secured. remove the axle nuts, I usually use a screw driver to keep the rotors from spinning and use an impact. Remove swap bar links. Either wrestle with the ball joint, or remove both control arm bushing bolts and pry the control arm out. Then remove the axle from the hub on both sides. I use the hood with a really long extension or long pole to put top pressure on the passenger axle and hammer on the bottom inner cv joint and it pops out. Driver's axle is the most tricky. Since you're dropping the subframe, check everything is supported again and drop the four main subframe bolts after removing the front little brackets. Now it's easier to get to the driver's axle and pry it out with a pry bar (one on either side works best). My least favorite part is reaching the last bellhousing bolt. It faces towards the transmission and is right under the rear exhaust. "easiest" way to reach it is to have a double swivel connector on a really long extension to an impact and fish it through where the engine to trans bracket went. With the axles removed this isn't that bad. Obviously have a transmission jack under here and have the strap or chain around the top to stabilize it. Remove the last bellhousing bolts, pry on the housing to separate and all you need is to clear the flywheel. Then drop and remove! Installation is pretty much reverse of install, but remember to fill your new torque converter (I don't recommend re-using one from a transmission that has blown like this one). When you get it installed, fill with about half the fluid, double check everything, start the car and dump the rest of the fluid in as quickly as possible. Then drive it till everything warms up and check the fluid again and adjust as necessary. First time I did this it took me a weekend, now it only takes a few hours. Plan more time than you need.
Hope that helps! Feel free to ask any questions along the way. It's not a fun job, but it's better than what the shops charge you for! My biggest hints are to get an actual transmission jack or rent one, and have two regular jacks on hand to drop the subframe (makes lining up the subframe easier on installation).
Also, rebuilding yourself isn't out of the question. It does take time and research and patience. Good luck!
