I've got Home on a separate partition so this will be interesting.
It should be pretty easy, then.
Just select "something else" or "custom" in the installer, so you aren't using the whole drive. And install everything to your current / partition.
When you reboot into the new system, you will have a new /home directory on the root partition. Now edit your /etc/fstab and add a line which tells it to mount the old partition as /home. If you look at your existing /etc/fstab, the line you need will be exactly the same as the line which mounts /home in your current file. You could just write that line down now. Alternatively, you can use blkid to get the UUID of the /home partition, and just copy the existing line for /, replacing the UUID and mount point (and changing the last number on the line, which is just the order fschk uses when checking the drives).
Reboot and it should now mount your old /home, and probably everything will work (at least going MATE to KDE).
In the event there are any problems, you can probably solve them by deleting configuration files in your home directory for the program having trouble. A problem also could be caused by the configuration files which are installed for every new user not having been installed to your old /home. Those files are located in /etc/skel. Just another place to look if you need to troubleshoot, but more than likely you won't have any problems.