No way-land
I'll have to see what the upgrade does for my non Wayland Pi 5, as when I tried a fresh install of Bookworm it was awful, so different to what I've been used to. I couldn't even get Wayland to change resolution from FullHD on a 1440p monitor and rsyslog wasn't installed by default - there is no excuse for that. I tried to install my usual Mate desktop and it completely ignored it, booting back in Pixel, even after configuring X.org. Hopeless.
From my very first Raspberry Pi Model B with 256MB, I've always upgraded the OS in place and moved the card to each new model of Pi I've bought. This worked through every OS version and Pi all the way up to Bullseye on a Pi 4B. I'd had the 64 bit kernel installed for sometime to play with 64 bit containers, but I was still using a 32 userland, then about a year ago I moved from 32 bit to 64 bit by taking a clean install on another partition, getting the same same packages on it as the 32 bit, then copying over the contents of /home and /etc, so not quite an in-place cross-grade but effectively ended acting like one.
After seeing how different a clean install of bookworm was on the Pi 5B, I decided, despite the dire warnings, to do an in place upgrade from the 64 bit bullseye to bookworm on the Pi 4 and I was pleased to find after following the instructions at PiMyLifeUp it worked perfectly. A few more steps than a normal in place upgrade, but I kept a working Mate desktop, rsyslog and all the rest of my setup tuned over the past 12 years, but with no Wayland or the other changes like getting rid of dhcpcd. It was then a simple step take that card out of the Pi 4 and put it in to the new Pi 5, to enjoy the 3x performance increase.