Hi, Welcome to the forum.
If by 'AU catalogue' you mean Australian Catalogue, then yes that is probably what you should do, attach to that via File / Catalogues / Open Catalogue. I think it may prompt you to convert it (to 662). This way Australian walls, plants etc and any customised elements or added objects, will still be there without having to re-import them. The Catalogues folder should automatically contain both Metric and Imperial catalogues, so you should be able to change them over.
This is the same with the Templates. If you open Settings/ Programme Settings / Templates, would should be able to use the browse button to change to either Metric or Imperial as the startup template.
You will also have to change Settings/ Programme Settings / Units of Measure to Metric if that has changed also.
Some of the options in Settings are project specific and others are global. If you find something reverting back for each new project, then you need to open a blank project, change the settings you want and save the BLD files in the Templates Folder and select that file as your new startup template. The customised settings will them be retained in new projects.
Hope that solves your problems
Allan