No, Ford will not write a Platinum PCM file over your existing non-platinum file. There is a lot of information out there that can be confusing.
What I have gathered is Platinum PCM programming might be slightly different and the ratings are using 91 octane most likely. If you put 91 or 93 octane in your non-platinum, your numbers won't be far off from a Platinum.
When you get into the Raptor/Limited F150/Stealth Expedition side of things, the engine itself, while the same displacement and externally identical, has different pistons, lower compression, oil cooling jets under the pistons, and different PCM programming at a minimum. I could be forgetting some other differences.
I have been looking to compare a platinum PCM file with my XLT file using HP Tuners, but have yet to find one to actually see the differences. That said, a few minutes with an MPVI3 from HP Tuners and a few table adjustments, you can easily get that extra 20 whatever horsepower from your stock file if you don't mind watching some Youtube videos, playing on the HP Tuners forum, and doing some reading on tuning.
If your willing to spend that money anyways, just save yourself a headache and time and just go get a tune/tuner from Gearhead, ZFG, or Boostking. You'll gain way more power and you won't sacrifice reliability. I have tried basically all of the big tuning companies out there and can say that Gearhead's newest stuff is perfect out of the box and needs no revisions if your looking for simplicity and the best refinement. ZFG and Boostking offer more of a "custom" experience with multiple revisions made to get the truck feeling how you want. MPT is also well liked. I would stay away from 5-Star unless your going to tow, but that's just personal preference. Many people enjoy their tunes.
Good luck.