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TDE is a fork of KDE 3.5.x, therefore it is not possible to install TDE and KDE 3.5.x on the same computer since many things would overlap.
If you migrate from 3.5.x, you should follow the instruction from here (but adapt the sources to buster). https://wiki.trinitydesktop.org/DebianInstall#Upgrading_from_an_existing_R14...
When you run TDE for the first time, a script will take take of migrating as much settings as possible from your .kde folder to .trinity folder. The original folder will remain intact.
Good to have backup before doing this kind of things, but if you wish to switch to TDE and still have KDE 3.5 installed, you could as well follow the suggestion from another user and use a separate harddisk for TDE in a dual boot system. Or just go for a clean buster install without any DE, then add TDE, copy over your .kde folder and you should be ready to go without conflicts or other things.
Cheers Michele