If the discoloration is mostly around the mat opening, then it may be mostly from the mat, not the art paper itself. If you used an "acid free" mat 15 years ago, the buffer may be exhausted by now, allowing the lignin (in the board all along) to do its dirty work. If the bevel has discolored, you'll know that's at least part of the problem.
Light exposure would be another contributor to the discoloration, especially if UV-filtering glass was not used.
The film on the inside of the glass could be from the old foam center board. 15 years ago the manufacturing processes were less sophisticated than today. Old-timers (so I've heard
) can remember opening a box of foam center board, and having the whole shop smell like plastic within minutes. It doesn't outgas like that anymore.
I would not suggest an invasive treatment such as deacidification. The aqueous solution might damage the image, as loose carbon from the pencil work could run or smear. Also, that kind of treatment might retard further acid burn, but probably would not do anything to help damage that has already been done.
Remount the art to a buffered alphacellulose board. ArtCare, with its zeolite "molecular trap technology" certainly would do no harm, and might help to retard further damage from chemical contaminants within the frame package. However, zeolite is a passive filter, meaning that it only traps what comes to it. That works great for contaminants swimming around inside the frame, but would not be as effective in removing contaminants already in the art paper. And when youy reframe this art, hopefully you will use inert products, which will not create chemical contamination in the closed-up frame package.
Of course, you'll use alphacellulose mats and UV glass this time, right? One more thought: If the pencil work is highly detailed, you'll like Museum Glass.