I would never change flap setting in middle of the T/O run: if it is so marginal, what do you reckon would happen in the case of an engine failure? As for changing direction at 15 kts: steering failure? The possibilities, which people here seem to have become complecant about, appear to have become meaningless at here. Never rotate at 47-50 kts unless type specifies it: you cannot prescibe a solution based on an average aircraft. Hogher flap settings (if put down in run) require greater T/O speeds remember, so the effect is cancelled out in effect due to the extra drag you will carry in the event of an engine failure, which is the first thing everyone who answered on this forum should have thought of!