What happens to my high level character when I log in to the free to play system? Will I be able to play it?

Depends on what you're expecting. Those old characters are considered "Gold" which can only be played ,as designed, with a paid subscription. You can convert them to "Silver" but you will almost certainly lose your custom powers and be forced to follow the set progression of powers . I only had a few toons at under 15th level and none of the archtypes seemed to fit either of them at all. e.g. I had a character named "Technofro" who was an African American Iron -Man type with a huge afro. None of the free types fit a gadgeteer/mech/inventor type nor did they have any darkness powers for my other character. I just saved the costumes, deleted the characters and started over with "SIlver" archtypes to check out what changed and how the free game works. I'm only 7th level right now.....using a Grimoire(supernatural/magic archtype)....it's o.k. so far.