By "in place" I took Bigwaff to mean that you wouldn't erase the disk as part of the installation process.
Something to consider is that apfs, the apple file system, has changed quite a bit between Mojave and Monterey to support the new signed-and-sealed system volume setup. I frankly don't know what, if any, problems you'd run into as a result. IOW, I don't know if Apple does some sort of conversion of the filesystem during the install. Maybe someone out there with more experience can chime in.
You could use Time Machine to back up your Mojave installation. You could then do a "clean" OCLP installation of Monterey (or later), i.e. you'd erase the entire disk during the installation. Towards the end of the process, Migration Assistant should run, giving you the opportunity to restore your user, files and apps from the TM backup.
Just throwing that out as a possibilty.