First go to the Assembly Navigator and select the Component of interest, press MB3, Properties and add a new Attribute, something like 'NEW_CALLOUT' and set it's value to the letter you desire, say 'Z' (you will see an attribute already there titled 'CALLOUT' set equal to the current callout letter, but don't try editing it as it will always revert back to what it's supposed to be).
Now go to your drawing and select just the Callout 'cell' of the Parts List of the Component of interest, press MB3, select 'Edit Text...' and when the Text Editor dialog comes up, on the Relationships tab select the 'Object Attributes' item (top row, middle icon), when the Target Object selection dialog comes up, select the what looks like another Object Attribute icon (this will show you only the attributes associated with the selected callout), select the Attribute 'NEW_CALLOUT', hit OK, hit OK again and the Parts List will update. The old callout letter will be reused, moving up by one letter, all remaining callouts (if you don't want that to happen, you'll have to lock them first).
Now if later you wish to 'undo' this, just reselect just the callout cell, press MB3 and select the 'Automatic Text' item. This will set that cell back what the system originally had assigned it.
Anyway, I hope this gives you what you're looking for.
John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
To an Engineer, the glass is twice as big as it needs to be.