How bad is it to accept a competitive Fellowship, then reject it BEFORE April 15
Let your ‘Yes’ be ‘Yes,’ and your ‘No,’ ‘No.’

It may be legal to accept and then reject the fellowship, but it does not make it right (in my opinion). Academia is still very much built on trust and goodwill. People who choose to scheme their way through loopholes in the regulations may benefit in a short term. But people who stand behind their word deserve ongoing respect and make for leaders of very strong academic teams. 

Don't do anything to people that you wound not enjoy done to yourself. If you are having second thoughts about this option, leave it and wait for something else which may be going your way.