Not a problem
Here's some further inspiration: http://carmensminiaturepainting.blog...or-helmet.html
If you pin the helmet through the neck; this way no seen areas of the helmet will be damaged once assembled. You can then work away filing down the rear detail, I wouldn't worry too much about damage to the helmet with some patience and smooth putty you can get there.
I'd try and place down a crude yet even area of putty once the detail is removed, allow that to set so you can then return to add finer detail without altering the base layer.
A lot of this might be obvious but naturally I'm unaware of your confidence with the old sculpting so I hope I don't seem patronizing