That depends; if the conditions of the fare purchased allow rescheduling, then yes, you will be able to reschedule the ticket. However, if you have already checked in or if your ticket includes special services, you will not be able to reschedule. 

If you wish to travel internationally, you will need a certain number of documents. The documents are: a valid identity card or passport, valid for at least 6 months before the return date, and in some cases, you will also need a visa depending on the country and your citizenship. 

Again, it depends. You usually don't need to pick up your luggage on flights with a stopover, as the plane just stops for a while at the airport and then continues its journey. You should always keep an eye on the signs, though, because if there is a change, you will have to pick it up.