When you say it refuses to do update, what do you mean exactly? Because even with the wrong helmet shape, the textures will still show up in game they will just look silly, the mapping will be wrong and so the top of the helmet will be over the visor, or something similar.
And it depends, each helmet texture will state what it's compatible with (usually they're compatible with about 3 different shapes) but the original GP4 helmets are only compatible with the original shape, and so if you want to update just 2 helmets, you probably need a whole set..!
Hope this helps
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"Anything can happen in Grand Prix racing, and it usually does."
"The lead car is unique, except for the one behind it which is identical."