I assume you had previously mapped the new commands Note Input > Lower Shadow Note by Step and Raise Shadow Note by Step (as they appear in the Key Commands page of Preferences) to the up/down arrow keys. When you did this, you presumably also made some other edits to the default shortcuts.
The commands that are mapped by default to Alt+up/down are Note Editing > Lower Pitch by Step and Raise Pitch by Step, and these commands are the ones that are responsible not only for lowering and raising the pitch of notes on pitched staves, but also for cycling through the playing techniques provided by a percussion instrument in a kit. So make sure that you have these commands mapped to shortcuts you are likely to remember, or perhaps avoid changing them at all, if you can, since Alt+up/down arrow is a pretty useful pair of shortcuts for editing music in Dorico anyway!