It would probably have to happen spontaneously, like in a situation where for whatever reason Barbarossa never happens but the Red Army then later bulldozes the Wehrmacht in a surprise offensive and the PCF leads an uprising. A bit difficult in that the PCF only took on their powerful role in the French Resistance because of Barbarossa.
I see a couple ways to get around this. Perhaps Barbarossa gets flubbed up earlier with a more competent Soviet response that means the Soviets lose less territory and people and are able to take to the offensive in central Europe earlier than OTL. Or the Soviets attack the Germans, but not in a lightning surprise at the very end of the war, but when there is still more than a year of war left and so the PCF has time to become resistance heroes.
Maurice Thorez is put in charge of the portion of France liberated by the Soviets.
I have a couple questions about knock-on effects. How are relations between the Communist administered French metropole and the overseas French Union territories going to develop?
The easy answer is to say that all of the French Union territories outside the metropole would become the refuge and and preserve of anticommunist Frenchmen.
But that is not guaranteed.
The maritime powers US and UK would ultimately have an advantage in those areas away from Europe, but it would not be instant. They would not have unlimited forces, most colonial areas would be non-priority (with key exceptions like North Africa), so at first the political sympathies of the local French administration, local French community, and native population would matter. Probably in some French Union areas Communist sympathizers, or people not wanting to split with the metropole despite its Communist dominance may be in power. Even among people not committed to the Communist cause, or going along with it, there may be French sentiment to not simply become puppetized by the Anglo-Americans.
Other non-communist French in the colonies, even with some wariness, would probably seek western ties for protection and sustenance.
However, where the French are pro-communist, local non-communist nationalists may be definitely up for regime change supported by Anglo-Americans, while the Soviets (though not able to invest much effort) would be supportive of continuity with France.
However, metropolitan French Communist authorities, and some of their loyalists in the colonies, may be ready to offer some key concessions and positions to natives. They would certainly authorize and encourage it in areas where the local French administration is anti-communist or Anglo-American backed.
In Indochina, how does Ho Chi Minh react to events in France? He’s the strongest player in postwar Indochina, and a Communist. Can Thorez and Ho work out a Confederation arrangement? Or a legal evolution from dependency status to a Franco-Vietnamese alliance of formal equals? Or will he split formally from Moscow and Paris and start seeking US, British and ChiNat support for an independent Vietnam calling its ideology something other than Marxism-Leninism?