Do I need a visa if I only change flights (in transit) ?
In most cases a transit visa will not be needed, but there are some exceptions to this rule because an example both the U.S. and Canada require you to have a visa when you only transit in their airports. It is highly recommended that you check the requirements with all countries involved in your itinerary for exact rules based on your nationality. Remember to also check countries where you have a 'technical stopover' which means you don't change flights where the stopover takes place but continue on the same flight you came on.
If your flight itinerary requires that you change airports during your journey, it is no longer considered a transit and you will have to check the applicable rules for a 'normal' visa.