Unlike drawings where an original drawing template can be replaced by a drawing sheet format, parts and assembly do not have this capability.
The only option I know of would be to use a "Drafting Standard".
If you have an "overall drafting standard" you can change that in the tools>options>document properties
that's really the main thing that is saved with a Part template. now if you have templates that contain models for start parts etc, this would not work.
Template can not be replaced, but by using "Design Checker" you can change most of the setting which you want in part/assembly and drawing documents.
very first you have create" build check" and create your customer standard which you want to set on existing file then run the "Check active document" option which will show the difference in your setting of drafting standard and ask for update your mistake wrt standard...
I've wanted to be able to do this for years and finally figured out a way:
- Mirror the part (ctrl-M)
- Check the box that says "break link to the original part"
- Close the original part
- Do a save as of the mirrored part, overwriting the original part (it should save it with the default part template)
- Delete the save-copy body that mirrors the part
I often receive models from customers or subcontractors that didn't use our template, which makes the part not work well with our drawings. This method gives those parts our default equations and custom properties that automate our drawings.