What I need to do:
78 portrait pages need to become 39 landscape pages.
E.g: Portrait pages 18 and 19 that are A3 should become one landscape A2 numbered p.8. Numbering and table of contents should reflect the change.
How I understood you:
I should link the Original Indesign Document inside new InDesign document page by page. In the new document, I will have two portrait pages that are linked on one landscape page. This new document also contains new page numbering (not linked).
I would select and group the spread page content for each spread, then change the document setup to A2, non-facing pages, then adjust the content position on page 1 and use Object> Transform again on the other pages.
Make sure no objects or layers are locked, you may need to address master page items.
Edit: if this is to be printed, you will need 40 pages.