For Frecce/hi-speed, mostly 10€: check if it is possible if it offers some´access my booking´ feature-or not. IF not-it MAY, perhaps be possible at the special last-min kiosks in the stazione (not all, mostly the main ones), but only before the original RES-timing, so they could use your seat for someone else. Never money back, only change of plans allowed. If this is restricted to same trip/other day or can even change to quite another one- I dk.