It's much worse than that. You'll erase everything, and likely won't be able to hold on to the new skills unless the plasticity was temporary.
Brain plasticity evolves in very specific ways. Simplified you could say that a young brain learns "short-term" things. It learns things (and learns how to cause things) that last tens of microseconds at and just after birth, then over the years moves to seconds, minutes, hours and longer.
But circuits are massively redundant. Until you lose about 90% of the part of the brain that fulfils a certain function, you won't notice a thing. Go over that, and ...
Every memory, every skill, everything in your brain is built on the basic premise that the shorter term neurocircuits work a certain way, that they have solidified. Every memory made when you were 10 years old is filtered through and interpreted by circuits established when you were 9. Memories made when you were 11, get filtered through the 10 year circuits, then the 9 year circuits, then ...
So if a neurocircuit changes that was built when you were 14 years EVERYTHING after that is gone. Every memory younger than when you were 14 years old gets filtered through the now-scrambled 14 year old circuits, which will transform it into noise, which the rest of the brain will promptly ignore.
This is why you regularly have demented people thinking they are much younger than they actually are, failing to remember they have kids, and so on. They suddenly start doing what they did at that age as well, leaving for work at 6am despite being pensioned for 30 years, in many cases to companies that have long since ceased to exist. Not recognizing kids (and especially grandkids). Note that this is usually emotionally very, very hard for those kids.
So if you successfully reset your brain to the plasticity it had when you were 14, every memory, every skill, every detail after that will disappear. The next day you will wake up, likely terrified, in a room you think you've never seen before, wondering where your mother is, not knowing what the hell a cell phone is. If there is damage that did not just do a one-time scramble of the circuit, but actually keeps it from re-solidifying, you will wake up like that every day of the rest of your life, not remembering the day before.
It also works in reverse : you can teach kids about long term effects (e.g. money) for months, and it just won't stick. Don't be fooled they'll be able to repeat the lesson back to you, but they can't apply knowledge that improves their long-term situation because it gets erased.