RAH,Robert...
We’re discussing films NOT finished in 4k.
To be clear, and presuming that the Blu-ray and 4k disc are derived from the same recent master, you’ll get a slight quality bump in image quality in viewing the 4k vs your player uprez.
They will not be equal, but can be very close. The Oppo does a superb job of uprezzing.
But the more obvious upgrade, which for many is the rationale for replacing a quality HD Blu-ray, will be the audio upgrade to Atmos or X.
I did that recently for the Pitch Perfect films.
Understood!
However, I did several comparisons to films not finished in 4K, but have received a 4K/UHD release. I compared that 4K disc to the previously released BD of the same title. With my Oppo 203 and untrained eyes, I can see significant differences on my LG OLED. I've done such comparisons on countless titles because of my former logic of not buying 4K/UHD releases in which the film wasn't finished in 4K. That was my mantra when I first got into the 4K format. However, those comparison differences changed my thought pattern on using that logic to determine my purchasing strategy.