It will stay the old format until you reach 4GB.
As I understand it yes. It will stay in the original RIFF format.
From the official info:
Projects saved in the older 32-bit format will remain in that format unless specific conditions are met, as detailed in the Help Center article."
Hope this helps 
FWIW, I created a Nuendo 14 project from an older template originally created in 12, and that resulting project could not be opened by Nuendo 12 after it was saved. Templates may work differently than older, regular projects from different versions numbers, but thought I’d mention it.
Tried the same here and it opened as expected in N12. As you say it could be the precise version number has an influence on this.
Are you an orchestral mockup composer (or as I like to say “one of the orchester people” 
) with hundreds of tracks in your template?
The switch from the old format to the new one should only happen on project files with a large file size as to my knowledge. It would be interesting to learn what other reason could cause Nuendo to change the format.
No, I’m a sound designer / mix engineer. I use a few VIs here and there, but 99% of my work is game audio development, linear post production, and music mixing. So far, Nuendo 14 is running really smoothly for me. That’s nice to see after updating from 12.
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Hmm… interesting. Thanks. I assume your template and project is not exceeding 4GB in size then? (Maybe a lot of ARA usage.)
If you have a nice text editor app you could open the .npr, that won’t load in N12, in the editor. Are the first four characters “RIFF” or “RIF2”?