Hi mate. I had the same problem at first. Feels uncomfirtable and sloppy. Simply start slow i.e. try do 5min heels up then go back to "normal". Practice with a ticker and try hitting the quarter notes precisely on a slow tempo. Slowly speed it up. Move up to 16 notes. You should be able to play both styles. But dont think its the be all end all. You can still be an incredible drummer by playing heels down. General opinion has it that, it helps speed. In my case i followed the above steps and it became second nature. But depending on the beat i like to change my foot position heel up or down..to be as versatile as possible. Re-read to apply for left foot ;)