This could be done using a EQ field or an ADVANCE field. The EQ field is probably the simplest:
• select the text to be processed and press Ctrl-F9. You should now see something like { selected text }
• Fill in the remainder of the field so it looks like {EQ \o (upper text,selected text)}, formatting the 'upper text' as superscript.
• Press F9 to update the field.

Note: Since superscripting ordinarily occupies part of the same space as ordinary text, you may need to increase the point size of the superscripted text and decrease the point size of the selected text. This could, of course affect the height of the line concerned. Alternatively, you might subscript the selected text. You can edit the field code by pressing Shift-F9 to expose its contents.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]