If you mean a Round 2 file, no, when you start a new round with cleared data, you are forced to play on the same difficulty.
The only way to play the game on Professional is to beat the game, and start a new game, you will not get access to any unlocked weapons (they are only available on a Round 2+ file, and you will not keep any upgrades (including health), but, you do get access to unlocked costumes.
Meaning, you can use the Special 2 costume set unlocked by beating Separate Ways, which makes Ashley invincible (and we aren't talking Boris in Goldeneye invincible). --- R.I.P. Eve English (Feb. 12, 1968 - Oct. 13, 2010) Momma Eve, you will be missed.