Doesn't happen else how would you land at a high altitude airport? We used to operate A320s in Addis Ababa at 7,800' elevation! It will take the FMS to approach phase which could be awkward if you do it by mistake in the cruise; to get back to the cruise phase just re-enter the cost index.
It's been a while and two new type ratings since I flew the Airbus so don't take my word for it and I can't give a reference but I'm sure that others will answer and either confirm of dispute what I say. Either way, enjoy the 'Bus, it's great!!
Happy landings
