You’ve begun with a misconception:

If nothing changed

In fact, a great deal changed with the signing of the Austro-Hungarian compromise in 1867. This agreement was developed in order to stabilize the Austrian empire, which had just suffered a disastrous defeat at the hands of Prussia in the Austro-Prussian War.

Since the death of Louis II at the Battle of Mohács, the Kingdom of Hungary had been ruled by the House of Habsburg as part of their collection of territories gained from inheritance and conquest, including Bohemia and the Lands of the Crown of St. Stephen, as well as Austria itself. Following the declaration of the Austrian Empire and the collapse of the Holy Roman Empire during the Napoleonic Wars, Hungary nominally became subject to a higher authority (the Austrian Empire) rather than simply being in personal union with the other Habsburg territories. After an abortive attempt at asserting national identity against Habsburg rule in 1848, Hungary lost much of its historical political autonomy and came under more direct rule. This period ended in 1867, when the Compromise restored much of the autonomy Hungary had previously enjoyed, including in domestic policy and the reesrablishment of the Hungarian Diet. The two countries maintained a joint foreign policy, however.

Austria’s status also changed from a unitary empire to what became known as the Dual Monarchy, as the Austrian imperial lands were separated from the lands of the Kingdom of Hungary.