SUMMARY
When editing an event, changing the details of the event makes the Apply button enabled, as expected. (Changing them back to the original values makes it disabled again.) However, changing the calendar (e.g. from a local korganizer calendar to a Google calendar or vice versa), in order to move it to a different calendar, doesn't make it enabled, even though it works when clicking the OK button.

STEPS TO REPRODUCE
0. Have more than one calendar.
1. Create an event in one calendar.
2. Double click the event to open the editing dialog.
3. At the top, select a different calendar from the combo box.

OBSERVED RESULT
The Apply button remains disabled.

EXPECTED RESULT
The Apply button becomes enabled.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20250108
KDE Plasma Version: 6.2.5
KDE Frameworks Version: 6.9.0
Qt Version: 6.8.1
Kernel Version: 6.12.6-1-default (64-bit)
Graphics Platform: X11

ADDITIONAL INFORMATION
I tested it in Kontact.