Cannot change the type of member used in the Q_PROPERTY without breaking compatibility

"Changing the type of a Q_PROPERTY is not formally an API or ABI break AFAICS, but it will still break every QML application that uses this property because int is natively representable in JavaScript and qint64 is not. The metaobject mechanism detects the types at run time. Re-compilation of user code doesn't affect that. It will still see that it's now qint64. We may have some interesting type coercion around that "fixes" the problem in some cases, but I wouldn't rely on it." ~ see this