Build xpages into a Domino Release 5 db? - or start fresh with Domino Release 12?

I'm updating a Notes client database built on Domino Rel. 5 to be an XPages application that will run in a browser on Domino Release 12.

Since the existing database has so many legacy documents, should I simply build my xpages in the existing database? - or, create a new database for new documents going forward and just use the existing database for referencing the old documents.

If I build on the same database, then it will make it easier to perform FT searches across all documents, so I can see an advantage there. However, I'm wondering if creating a new database running on a newer ODS version would be the better option.

What's the best approach here?