You can change them between 1,2,3,..., A,B,C...., or choose to show nothing.
But I'd leave the letters there, then put those alternate titles in Row 1. (Use
alt-enter to force a new line within the cell).
Then make sure A1 is visible
select A2 and then
Window|freeze panes
So row 1 will always be visible.
(And you may want to use File|page setup|sheet tab|rows to repeat at top when
you print.)