No it will not erase data if done by:-
To convert your drives to NTFS:
Right click on 'my computer' and select 'manage'
From the computer management window, expand storage and select 'disk management.'
Using the 'file system' column of the upper pane of this window, you can easily check what file system each of your logical drives is using. Make a note of this information.
Now open a command prompt window by going to 'start\run' and typing 'cmd'
To convert a disk to NTFS, type 'convert (drive letter): /fs:ntfs'
So for example, if you were going to convert your C: drive, you would type 'Convert c: /fs:ntfs' at the prompt.