When I apply a template to a drawing, properties from my old library appear!

This type of behaviour appears when you apply a template that you made yourself using another library. Since a template is just a basic drawing that is merged into the current drawing, it will bring in any properties that it contains.

For this reason, you should make sure that...

1. NO library is selected when creating templates, so that no properties irrelevant to the template are saved with it.
   
2. You need to start with a totally blank drawing page, so that means you should not apply other templates before starting on a new template either.

See tutorial on Templates. If done correctly, then any template you apply will only import the bare minimum of properties that it requires, and so will always be compatible with future libraries.