The chips in a TI-83+ are different from those in a TI-84+.
You would need to find the replacement chips, figure out how to solder them into place (They aren't the same size, or in the same places, so good luck) and then wire a new USB port in, along with its controls.
The architecture (the style of stuff used inside) is similar, but the designs are different enough to make this idea impractical.
Why do you want to do this? For the extra archive space? For the extra speed? For (please not this) MathPrint? There really aren't any other reasons than these, and all of them entail more complications than would appear at the surface. Let's go through them
1. For extra archive space, you would need to make 2 changes. First, you would have to physically replace the Flash ROM in your calculator with a larger chip, ideally already programmed for this purpose. This requires that your current TI-83+ has one of the hardware revisions that has a separate Flash ROM. After you got the new chip into place, you would need a modified TI-83+ OS that would recognize the extra space and use it. That's probably not a very complex modification for an experienced OS modder, but that doesn't make it easy.
2. For speed, you are pretty much out of luck. If you have a TI-83+ SE, you already have the speed of the 84+, because it also included the more advanced timing architecture. However, the TI-83+ has no extra timing hardware and nowhere to put it. You would also have to make extensive OS modifications to support any modifications you did make, since they would probably not be close enough to the real 83+ SE to use the same OS.
3. MathPrint is not good. It slows everything down for a little bit of pretty text, and it's extremely unstable. It crashes a lot, and it makes lots of BASIC programs incompatible across calculators. I don't know why you would want it.
