I'm going to chime in with a few comments.
1.) There's really no such thing as an 'in place upgrade'. The Windchill application has to be installed in it's own new folder. This new installation will be completely separate from the previous installation, even if located on the same physical server. It's not possible to update the files in the existing Windchill folder during an 'upgrade'. The only times the existing installation (in the same folder) can be updated is during a critical patch install or during a maintenance release install.
2.) 'Upgrading' Windchill really means to install a brand new clean Windchill system (and empty database), then overwrite that new database with the one from your old system, and finally run the 'upgrade manager' process to update this old database to work with the new Windchill system.
3.) It is possible to install many different versions of Windchill, or even multiple copies of the same Windchill version, on the same server at the same time, but only one of them can actually be running at a time. If you configure Windchill to run as a Windows service, you will need to reconfigure which installation gets started by by the service. (I find it easier to not configure Windchill as a service until I'm done jumping back and forth between the different versions.)
4.) Windchill needs to talk to a database. You can configure each of these separate Windchill installations to talk to the same database instance, or you can configure each one to talk to its own separate instance. If they are all talking to the same database and same instance, you will need to back up and restore the database depending on which Windchill installation (version) you want to run at that moment.
5.) Windchill 11 stores a bunch of stuff in the local LDAP (Windchill DS). During the upgrade process, a bunch of information is going to get moved from Windchill DS into JSON files and the database. Since you will probably be running this upgrade process multiple times for practice, it's important that you have backup of the LDAP before you attempt the upgrade so you can roll it back to the starting condition for subsequent upgrade tests.
6.) You can continue to use the existing Windchill DS installation for the Windchill 12 LDAP connection. You don't need a separate LDAP installation. Just make sure you back up everything.