As the manager of three Cottman transmission locations, I can safely say it does not necessarily mean you need a rebuild. Unless you're in central TX... then I recommend a Platinum rebuild with new axles and wheel bearings*[
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Burnt fluid means burnt fluid. Auto trannys get hot. They use friction clutches that are held together by hydraulic pressure and are bathed in ATF as well. Once you're in gear and going, theoretically there is no slipping or wear between those clutches, but just like with a manual tranny, during shifts is when those clutches are being applied and released. That's when they slip. They make pretty crazy localized heat. After years of wearing off friction material, the fluid becomes abrasive and accelerates wear... which is why fluid changes are recommended.
At this point if the transmission is working properly, pull the pan, change the filter, and go about your business. Some people say you shouldn't do a tranny flush, but I don't stress too much about it. Pulling the pan will replace about 4-5 quarts of the total 9 or 10 that is in there. Doing a flush will replace it all which can be a bit of a shock to the clutches (but that is another lecture).
I do flushes on cars with less than 100k (depending on the transmission and use) but stick with pan only on older or HD vehicles.
I won't sugar coat this... the 5EAT and 4EAT transmissions in the Subarus are not the greatest. The fact that you are experiencing burnt fluid at 100k is perfectly normal but it should be addressed.
You might be surprised at how some of these modern automatics suck. Jags are lucky to make 60k. Rovers you're lucky to make 40k. 98-05 Hondas are often good for 100-150k. 02-07 Maximas? Fuhgettaboutit. No parts available. 9 times out of 10 you have to buy a new tranny from Nissan for $3000 and they fail about every 60k.