Drum Machine Designer is essentially Ultrabeat in multi-output mode, and saved as a track stack, where individual percussive instruments are represented by their own tracks. When you browse library presets for Drum Machine Designer, these are saved as summing stacks – groups of tracks where each would have its own channel strip setting. In order to be able to switch the sounds, you'd have to unfold the stack and find the track with Ultrabeat plugin.
The first track in the stack would normally contain the Ultrabeat plugin, which you then can work with to switch the sounds. I assume you know how to operate Ultrabeat. And if not, you may read up on that here: https://help.apple.com/logicpro/mac/10.4/#/lgsifdc04988 > "Ultrabeat Assignment Section" (Import sounds or EXS Instruments).
In short, you won't be able to automatically reload the whole kit when Ultrabeat is in Multi-Output mode. Thus, you'll need to replace each sound manually, using one of the methods for importing sounds into Ultrabeat (as described in the help article linked above).