Focusing on Hong Kong, I'd imagine the authorities would probably be sufficiently spooked to demand reinforcements for BFOHK to say the least, but regarding other effects that probably depends on what we're considering "the next few years," given how fluid the political situation in China was. Certainly I think we're likely to see at least a toned-down version of the 1967 communist riots, since the OTL 12/3 incident was the primary catalyst for that campaign of violence, but if the takeover happens before 1966 the situation might get defused faster with Liu Shaoqi still in power and Zhou Enlai not yet being put on the tightrope.
Politically though, I do think it very likely that we will see democratisation happen. After the failure of the Young Plan and the Communist takeover of the Mainland, any further moves by Whitehall and the colonial government to introduce universal suffrage elections were met by Beijing with threats to invade and so they never happened until the 90s. China invading Macau would send a signal that those threats would probably be irrelevant in the face of them actually invading a neighbouring territory that had accommodated a pro-Beijing business elite.
What other implications there might be, especially considering the massive intake of people fleeing the Great Leap Forward on top of the nearly 2 million who arrived after the Civil War, I'm not too sure. I think it's likely that KMT-aligned groups would take it upon themselves to escalate violence against pro-Communist ones in retaliation, leading to a series of clashes echoing the 1956 Double Ten Day riots. The instability that would entail could lead to various outcomes - Beijing finding a casus belli and taking advantage of BFOHK's distraction to invade, a spontaneous White Terror initiated by the pro-KMT camp, the 1967 riots meets The Troubles, Britain giving effective control to the Communists like the Portuguese did after the 12-3 Incident. All potential outcomes that would cause a crisis in Hong Kong's future far worse than that which spawned the migration wave of the late 60s and early 70s.
