Is it possible to change all the flat notes in a large score to the equivalent sharp (enharmonic) notes - and/or vice-versa?
I suspect not, but it could be a useful feature. Maybe someone has made a plug-in.
However there is a way which will work some (most) of the time. Select the passage to change, Use the down arrow key to move it one semitone down, then use the up arrow key to move it back up one semitone. This should change flats to sharps. To change sharps to flats use the up arrow key first, then the down arrow key.
This method will work as long as the section to be modified doesn't include both flats and sharps. I don't know what it does about double sharps or double flats - I don't use those much!
It should work on very large selections - but then there might be a need to check for anomalies in the whole end result.