Here are some things to consider if you want to change stops by yourself during a performance in public:
1. Change stops by hand only in places where at least one of your hands are free.
2. If you need a quick change, plan it so that you pull or push only a few of the stops at a time.
3. Consider which hand is free for this change and choose the stops on that side of the console.
4. Don‘t grab more than two stops at a time as some instruments have a heavy stop action.
5. Don’t take the registration changes for granted. Practice them ahead of time repeatedly.
6. Some pieces with large amount of stop changes will work better on organs with combination action so choose the repertoire wisely.
If you don't have access to an actual instrument on which you will be performing, contact the local organist and get several detailed pictures of the console and pedalboard. This way you can plan your stop changes away from the instrument and you will not waste any precious time when you have the rehearsal.