Import / export resource assignments

Can you help with this:

I wish to import resource assignments from one project to another. The activity IDs etc. will be the same.

In the destination project, some of the activities will be actualised (but no resources), whereas in the host project they are all planned (i.e. all remaining units).

I've tried all methods including exporting the original to XER, XML and Excel and similarly importing, with all kinds of import combinations (keep existing, import new etc.). This is on my own laptop with a standalone installation using P6v8.3

Help otherwise I'll have to do it manually - very long job!!