A tt fit requires more than flipping your seatpost forward. You'll also have to mess with your stem, and possibly replace it.
The cost of a set of clip ons, a new seatpost, and a new stem isn't much compared to the cost of a TT bike, but the time required to move back & forth is quite substantial.
Still tho, if you race in position A, training in position B isn't going to make you any faster. (than training in position A would)