… I would like to transfer (copy) my scores into a new template …

This sounds like you are trying to give your scores the formatting of the new template.

… but when I do that, all the formatting disappears and I have to redo the layout which takes a lot of time …

This sounds like you are trying to give the new template the formatting of your scores.

* Are you trying to create a new template that has the formatting of your scores?

So that all new documents, created from the new template, will have the formatting of your scores?

Please explain.

Thanks Peter !

I am trying to give my old scores the improved document options of my new template (stem lengths, beaming, flags, music spacing options etc) while keeping the layout of the original scores. 

Either I need to import all those new document options into the old scores (but not one by one please !) or do the opposite and import (copy) my old scores into the new template but keeping the original layout. 

… I am trying to give my old scores the improved document options of my new template (stem lengths, beaming, flags, music spacing options etc) …

… I need to import all those new document options into the old scores (but not one by one please !) …

There is no need to import the document options one by one.

In your case I would

1) Open the new template, and save a Library of its Document Options (stem lengths, beam settings, flags, music spacing options &c).

2) Then, load that Library into the old score.

In this way all the document options will be loaded “at one fell swoop” (not one by one).

Because of the new spacing options you will have to

- Apply Music Spacing,

- Update Layout.

That is exactly the trick I needed. Seems so obvious now. Brilliant. MANY THANKS !