Did he really change his mind? I doubt but regardless, him claiming all their family properties back has nothing to do with his own personal mindset of whether to push for German monarchy again or not.
It's their family property after all.
As far as I understand it, certain parts of the property of the Prussian Royal family were considered to be "private property" of the family in a contract between the family and the then reigning government in 1926. After 1945 the Sowjets took the possessions away that were in their part of Germany. Now the family tries to get these objects and places back.
The problem is that a judge has to agree that the former owner of the belongings, Crown Prince Wilhelm of Prussia, had not been responsible in a strong way ("wesentlich" in German judical language) for the ascent of Hitler. The government of the German Federal State of Brandenburg (in which the places like Cecilienhof are) had stated that of course he was because he had helped Hitler with public audiences, invitations, even a press release asking the Prussians to support Hitler. The family states that while he did this, it was in now way "wesentlich" for Hitler's taking over the power. The Crown Prince had not been a member of the Nazi Party, after all. So now we have experitises by two reknown historians who are on the same side when it comes what actually happened. But on two different sides when it comes to what the Crown Prince's actions actually mean.
When it turned out to become a yes or no-decision by the court, the Head of the Hohenzollerns started negociations with the government of the Federal Republic to get back some possessions, effectively stalling the court proceedings. But as now, after years of negociating, no result is coming out of the proceedings, the Prince of Prussia added up all the family really wants back and is threatening now to take the issue to court and to wait of the judge's decision.
Probably to get the state's negociators to come up with real solutions.
Thus far, no new court proceedings have started, the old one against Brandenburg is just bits & pieces compared to the majority of things that were granted to the Prussians in 1926, we'll see what will happen.
Other Federal States have in fact given some palaces and art objects back to their Royal families but not all families are content with what they got. (See Baden-Württemberg and the Margrave of Baden-case, while AFAIK the Duke of Württemberg was content with his negociations.) But then there wasn't any Sovjet interaction after the war and the Prussians are not the only family to still try to get things back.