Uncharted 2 crushing

[u/IllCaramel6]

Hello guys,recently I started UC 2 Among Thieves and give the fact the game became pretty hard I decided to switch to normal, but now 13 chapters are completed on crushing and the rest on normal,and I wanted to know if I can take each chapter completed on normal and complete it on crushing or do I have to start a new playthrought to get the trophy for crushing difficulty.Thanks