Setting a stock vesting schedule retroactively

So here's the the quick summary:

Three co-founders with shares equally divided. Started working on it about a year ago, got incorporated about 4 months ago. Now one of the co-founders wants to leave.

We haven't set up a vesting schedule, but agree that that would have been an ideal move. Now we want to know if it's possible to basically retroactively set a vesting schedule. As in, give shares back to the company, and reassign them on an agreed vesting schedule, upon which the leaving co-founder will immediately stop vesting. Is that possible, legally? Or you can't set up a vesting schedule starting last February (2012) if you already incorporated in 2013 and there's nothing in the documents?

It's perfectly possible to set up the vesting schedule.

Typically with founder's shares, you had to buy them for cash at a price per share (typically a fraction of a cent per share). The way you execute vesting with founder's shares is to set up an agreement that the company can buy back the unvested shares at the price you paid for them.

So in this case you would figure out (i) what the vesting schedule is you wanted (which can easily be before you registered the company) and (ii) how many shares are vested by now for the person leaving. He or she would then agree to sell back the unvested shares, as a straight buy/sell with the company, with no reference to vesting.

For the rest of you, you would figure out your vested shares up until today. You would then sign a vesting agreement (i.e., contingent share repurchase agreement with vesting) for all your unvested shares going from today forward.