Final Cut, in general, tries to operate in a non-destructive way. As Tom Wolsky writes, what you are seeing is the output of the timeline which FCP will want you to render since you changed the codec. At that point, you are seeing ProResHQ after FCP has created a separate render file that it’s playing back from. Since ProResHQ has a higher data rate than ProRes, you will not likely see a difference. It’s a little like taking a VHS tape and dubbing it to digibeta. You won’t really lose anything in the dub, but it’s still going to look only as good as the original.
In order for your experiment to yield useful results, you will need to take a file with a higher data rate than ProResHQ can handle (like 2k). Then convert from the original file into both ProResHQ and ProRes using Compressor.
Also echoing what Tom wrote, it’s unlikely you will need all the bandwidth that HQ provides unless you’re dealing with some really high end original footage (like film). For local TV broadcast, I’ve been very happy saving space and using ProResLT. It’s still a higher bandwidth than the transmitter uses. I just make sure not to go tossing out the original footage.