OK...some of this is from my memory. It's been a while since I tore into a 998.
To attain reverse the transmission applies the front clutch and rear band. The front clutch also applies when the transmission shifts into third gear. Do you have any problems while driving it above about 30 mph...such as slipping in high or third gear? When it does shift into third does the engine rev up before it shifts?
The rear band also applies when you manually shift the transmission into first gear. In the drive position the function of the rear band is handled by an overrunning clutch when you move out from a standing start in first gear. Therefore you're not likely to notice a problem when driving forward from a standing start if the rear band turns out to be the problem. It's my experience that the rear band seldom goes out or even needs adjustment.
The rear band is adjustable. You can get at the adjustment by draining the fluid and taking off the pan. If you're going to adjust the bands post up a message and I'll go into more detail. The clutch is not adjustable, and if that's the problem you're looking at pulling and disassembling the transmission.
There's no way to know for sure where the problem lies until you do pressure tests. However, here are a couple of thoughts:
1) You've already done the first step toward resolving the problem...changing fluid and filter. I'm assuming your fluid level is between the full and add marks. Check it while it's warmed up and with the engine running in the neutral...not park...position.
2) You can eliminate or confirm a rear band problem by driving the car forward while in the "1" position. Get it up to about 15 mph and then let off the gas. If the car simply coasts and engine speed falls to idle you have a rear band problem. If it slows down against engine compression the rear band is OK and you have either a front clutch problem or a hydraulic pressure problem.
The good news is that the 998 is a simple transmission. Anyone with basic knowledge, a bit of experience, a decent set of tools and a good technical manual can successfully repair this transmission.