In many areas of FIMS you will find case insensitivity. This is done so that codes could be entered as abc,ABC, Abc, etc and still be matched or selected as the same value when doing reports. This does cause issues when trying to fix the visual.
The trick to forcing a save is to add a character to the field value being changed:.
If the field currently has Mcduff, change it to McDuffx. Because mcduff is not the same as mcduffx, the new field will save with the correct case. A further change of McDuffx to McDuff and you have changed to the new cases.
This does seem tedious, but it is an infrequent occurrence and it does mean that the data entry requirements for the codes can be less stringent.