Upsampling without changing the size of an image

[AngrySalamander]

So this isnt for a project but I want to understand how this works for future printing jobs. In the attached screenshots of a photoshop file I have an image thats I'm upsample from 72ppi to 300ppi while keeping the dimnesions the same. Lets say, for the sake of this question, this image was a tiny 6"x6."

My question has 3 parts:

1) How does this affect printing quality if scaled up to a standared print size (A3,A4...etc) after upscaling?/ How does this affect printing quality in general?

2) Is the 100% zoomed preview showing how my image will look if printed and if not does the prview window even matter?

3) Does illustrator show a good general view of how something will print with the "Overprint View" on?

Thanks y'all. If you need anymore screenshots/ info let me know

 

 

[Ton Frederiks]

Upscaling will always add new pixels. Photoshop can do a good job generating these new pixels, but there are limits to upscaling.

If you set Preferences > Display Print Size at 100% Zoom you will get a good preview of the quality.

Overprint Preview should only matter when inks are set to Overprint.