In my opinion, be clear and obvious
I get your point. You want to let your character shift slowly under the radar without the other players noticing before it is too late.
It would work awesomely in a book or a movie.
The problem is, I have seen tons of these attempts as GM and most often they fail. The typical reason for this failure is that not only did the other players not notice, they noticed none of it. When (If ever, some players sit on their characters secrets all campaign and they never gets revealed) the secret finally gets revealed, the result might well be "Huh, what? Strange." rather than the desired effect.
My advice would be to consider what you want to get out of your characters development. Do you want to tell a story? Do you want to surprise the other players? Do you want to get a certain emotional reaction?
Whatever you wish to accomplish, the route to get it is probably not for you to have certain internal experiences (in your mind when you roleplay) but rather for your fellow players to have certain experiences. Focus on those. Do the entire transition in an obvious way. Make the starting state believable, show and tell (the other players do not know what your character feel unless you tell them or show them) every transitional event that shapes your character.
This way, when the finale comes, you will know that the other players are on board and pays attention.
This is theater. You don't have to tell them the script in advance (you should not), but make sure every step of the way hits the mark in an obvious way.
On telling the GM
You should definitely tell your GM about your plans. At least as a general outline. There are three good reasons for this:
- The GM can help you. Make sure opportunities for certain scenes happen, leave clues to the others that do not appear to come from you, etc.
- The GM probably has plans of his own. Give him a chance to bind your story to his, otherwise you might well ruin all his carefully prepared campaign-work. In concocting a story like you are, you are in part doing GM-work. As such you should make sure to work together.
- The GM does things like this all the time. Villains with a secret plan is not very interesting if they take their super secret plan to their grave. Since the GM probably already have encountered this issue, he might have great tips to give you.