If I have the Ford Performance Power Pack 1 installed right now is it possible to have a tuner just change the just the rev limiter and speed limiter but leave the rest of the tune the same?
Yes, you can change as little or as many parameters in a tune as you want. There's no minimum to be able to flash a tune, so you can change a single parameter.
Be aware though that most if not all tuners would not just simply take your existing tune and adjust one thing, they will want to write you tune of their own which will override the entire PP1 tune. And on that, you'd have to purchase their full tune which is usually $2-300 and some sort of handheld device which is usually $3-400. So you're realistically looking at spending upwards of $700 for this.
Personally I would dump the pp1 tune and go with a reputable tuner, Lund, my favorite, Palm Beach, aed, etc. They no not have the same limits that the frpp tunes do and can truly push the car to what it is capable of.