Greek dictator George Papadopoulos blown up sky-high by Alex Panagoulis in August 13th 1968. OTL Papadopoulos survived by mere seconds as either Panagoulis slightly misjudged by a few meters the distance of the dictator's fast moving cars or the detonator slightly delayed. ATL Panagoulis is that second or two faster and the dictator ends up dead.
This does not kill the dictatorship but puts it in very severe trouble. For one thing organized active resistance to it, while there was disconnected and was not getting active mass support to the extend that the regime looked too strong and her propaganda mechanism could suppress bombings by the Democratic Defence. But Alexander's little act of tyrranicide changes this in one fell swoop, it proves there is active resistance and said resistance is having major successes. The ranks of the resistance both within the army and among the civilian population will be massively increasing. Then the regime will be facing major internal trouble. Someone got to take Papadopoulos place as head of it and there is no generally acceptable successor. The knives will be out among the various factions of the junta... at the worst possible time for it as resistance will be mounting. In November 3 the mass demonstrations triggered in OTL by the funeral of George Papandreou Sr are even worse triggering street fighting for days in a parallel to the 1973 Polytechnic uprising.
So within the next 12-18 months, call it by March 1970 the regime is gone, probably down to what was in OTL to become the failed Navy revolt plot succeeding with mass popular support and triggering open revolution. Hard core elements of the junta are arrested or killed resisting arrest. Some of the most notorious like Ioannidis end up in court martial and executed. A civilian government under Panagiotis Kanellopoulos and George Mauros leads the country to elections and a referendum on the future of the monarchy leading to the Third Greek Republic by the summer of 1970. The actual form of the third republic constitution may well be significantly different, instead of Karamanlis effectively copying the 1952 constitution under the pressures of the time in 1974, the ATL constitutional assembly will have significantly more leeway. Some short of presidential republic is not unlikely and proportional representation for the parliament is rather likely.
In some significant ways this third Republic is rather different to the 1974 one. First democracy was restored by popular uprising with the armed forces playing a leading role in it. Second the resistance organisations can't be dismissed that easily ATL and they were dominated by centrists and right wingers. While the Communist party will be probably getting legalised in the ATL just as OTL Greek politics are likely to be tilting much more to the centre and right than the immediate post-junta years. PASOK will be coming to being and given Papandreou will be getting significant but shredding the Centre in half and taking over most of it is much less likely particularly with proportional representation around. Most likely you see evolution similar to Cyprus with a strong right wing party, the equivalent of OTL New Democracy, a strong centre party or coalition of centre parties and a strong socialist party, PASOK absorbing the euro-communist leftists. Plus fringe parties to the left, KKE will be starting strong at around 10% but falling to about 5-7% as the years pass while in the opposite side of the spectrum some royalist/pro junta party is likely to be getting a consistent 4-7% of the vote as well.
Thoughts?