This is very easy to change. Since the car's been out in the elements, (thus the dryrot and cracking), I'd suggest a small does of PB Blaster applied to the small screws that hold the sweeps in place.
Just roll the window all the way down and pull (gently) toward you (ie: toward the inside of the car) you will see about 5-6 screws (can't recall the exact amount). Hit them each with a small shot of the break free spray (maybe a few times/days before doing this. I recently had to take some off an older door that was off the car with no glass and they were rusted in. Even standing on the door and leaning into it was hard work. So, that is why I recommend the break free a few times and maybe a few days in a row before doing it. The last thing you need is a super stubborn screw that won't come out and the only tool you can use is a tiny "L-shaped" philips screwdriver (there's almost no leverage...)
Other than uncooperative screws, this is very easy to do. (no need to remove the door panels or the glass). Good Luck.