Qt3 to Qt4 required you to change your code to support Qt4.
There will also be porting needed for Qt4 to Qt5 (e.g. QtWidgets being a separate module, changing QtGui include to QtWidgets). See http://qt-project.org/wiki/Transition_from_Qt_4.x_to_Qt5 for the full list of changes.
I used QAudioFormat from QtMultimedia in a project I was developing, This got moved from the QtMultimedia package to the qtmobility-dev package and from the QtMultimedia folder to the QtMultimediaKit folder.
Are you going to use the Qt Q... variants or the KDE K... variants of the different classes? Oh, and you cannot use them interchangeably (e.g. the file open/save dialogs).
Also, there is more emphasis in Qt on the QtQuick platform which is another moving target. Even then, are you going to use pure QtQuick, or use the new Ubuntu Phone/Tablet APIs they are creating?
Waste not, get your budget cut next year.