Wow. This is a tricky one. I do not believe you can move forward without making it into a pre-comp. Making it into a pre-comp, in general, will still be beneficial as you can make changes to the original. I might suggest turning it into a luma matte and then use minimax to expand/contract the non-alpha channel. Add a normal tint from there. If you want to change all of the linework in general, put a solid that’s the same colour as the colour that you want to make your linework under the layer which has the linework keyed out. You could also duplicate the layer without the linework, and then blur the one on the bottom. If you don’t want it that soft, use the first method I told you. Maybe combine the first one and the last one. If that doesn’t work, I don’t know what will. Hope that helped.