@gGeorg: Nonono!!! Do not delete the bus line! Read the guide for instruction on replacing the fleet: http://steamcommunity.com/sharedfiles/filedetails/?id=311812980
The moment you delete the bus line, the peeps will reset their travel pattern and even cancel their destinations, because the system will identify the destination as inaccessible. The new busses take a while, but not 20 minutes to do a full line cycle. Usually 5 or 8 minutes, depend on how long the line is.