Quality loss when upscaling in Premiere, but exporting original resolution.. ?

[RunNGun]

So, not-so-hypothetical situation here...

Say I shot some talking heads in 4k, drag them into a new 4k timeline, but I wanted to add some 720p be-roll. I drag that broll into the 4k timeline, scale it up so there's no black around the clip, finish my edit, and export in 720p. I've effectively uprezzed the 720p broll to 4k in order to match the timeline so it doesn't look wonky for coloring and reviews etc.. but I'm exporting the project in 720, so it shouldn't matter (or lead to quality loss), correct? 

My assumption is that Premiere re-references the source file before export.. so any upscaling quality loss in the timeline goes away if exporting the same resoluton as the source footage. 

Example: You're dragging 360p footage into an 8k timeline (it's going to look terrible once scaled up), but you export in 360p... should look fine, right? 

[R Neil Haugen]

That would be easy to test, of course. Just do that with a three clip or so sequence real quick, export, take a look.

[RunNGun]

Tried that... downloaded some old footage from the Library of Congress, opened a 5k timeline, dragged it in and scaled it, exported in the original (source) resolution. By my old eyes, doesn't look obviously degraded... but this is obviously a very non-scientific method. 

[Averdahl]

...but I'm exporting the project in 720, so it shouldn't matter (or lead to quality loss), correct?

By @RunNGun

If you scale up 720p to 4k and then do an export to 720p, the 720p footage you scaled up will lose quality since you scaled it up to 4K. It will never come out "untouched" by the scaling. This is the way the render pipeline in Premiere Pro works.