Hi Mark,
Switching from Geographic to UTM is as simple as clicking on the UTM icon next to Geographic icon in the Calculate tab... This said, if you have done some Plan drawings while using Geographic "projection", you won't see them anymore when zooming onto your displayed data. There are not gone, but all your Plan drawings will found around X=0, Y=0 as the cave ref point in Geographic "projection" is X=0, Y=0... far away from your cave ref point in UTM coordinates.
To bring your drawings to the correct UTM location, first under Drawings tab, enable the Grid display. You should now see the UTM grid and coordinates under the Elements tab. Zoom out until you have both X=0,Y=0 (where your Geographic drawings live) and your cave data displayed in the UTM projection on the same screen. Select all the drawing elements at once and drag them toward the UTM cave ref point. Zoom in and drag them again, closer to your data.... Do it again and again, until you can drag them to overlay nicely your survey data.
This would be the same "issue" if you decide to switch the depth ref Entrance/Sea level.