iPhone 15 Pro case compatibility with iPhone 14 Pro

Simple answer: no.

Even if they physically are the exact same size, the placement of various controls and the phone's camera would be different.

You can see for yourself here: iPhone Manuals and Downloads - Apple Support

I would expect not

if one goes to a case

Tech21 FlexPro for iPhone 15 Pro (MagSafe compatible) - Clear - Apple

and scroll down to the text compatability and it only list iphone 15pro then I would never risk purchasing it if I had an iphone 14 pro or the other way around