Please add to your to-do list: the ability to change rehearsal marks on the fly, e.g. when an older score’s sequence skips the letter I (F,G,H,J). At present you apparently can’t do this. If I add a second mark to the same bar (getting J) and then remove the offending I, the J changes back to I. 
Select the rehearsal mark and look at the Properties panel (Ctrl+8) to show it: there you can manually override the ‘Index’ number, which will override its order in the sequence.