That is true, after Apr 1 its winner take all in the states.
I think it is optional winner-take-all after April 1. Prior to April 1, a state gets hit with a 50% penalty for having a winner-take all primary. At least that's how my RNC Committeeman, Bill Crocker, explained it. Supposedly the TX GOP is looking at changing the delegate allocation rules to take advantage of the timing. If the DemocRATS hadn't challenged the redistricting, we wouldn't have the possibility of a winner-take-all primary this year.