I have a Vintage conn 5h that is friction fit and would like to know how feasible it would be to get it modified with threading and a screw so that it can be the standard screw fit? Also the slide is a bit scratchy 7/10 and has a small hole but it doesn't affect the sound too much.

That horn is around 100 years old. Yes, the slide tenon and receiver can be replaced , using modern parts. The inner slide tubes look like either raw nickel silver, or, rough nickel playing (either of which would be appropriate for a horn this age)

What is the serial number? Can you show pictures of the slide stockings?