I think it would be an enormous change. Just look at how much more harmless things through history has changed everything. Some monk nailed his opinion on the church door and everyone lost their shit. It is probably the best parallel to an UFO disclosure... Luther's harmless opinions resulted in the downfall of the Christian unity in the west and religious wars and so on, and that was just about the question about authority, this is more like as if he had proven that the Pope was the devil, or something else that destroyed the whole foundation for their faith and worldview.
I'm sure we'd all agree that proof that a UFO was in fact an ETI artefact would be
enormously scientifically important. It would be of great interest to many people. Many astronomers etc. would probably have to revise their views. Many textbooks and popular science guides would have to be updated.
Wider social implications in the longer term are less certain, I think.
Convincing (e.g.) photographic/ film evidence by itself might be sufficient to demonstrate that a UFO might be an alien craft, but not tell us much else. It would demonstrate "we are not alone". The biggest practical implication would be that a much more technologically advanced culture has the ability to visit Earth- and that they have little interest in establishing a meaningful dialogue, which is concerning.
Luther's 95 theses (he was a priest, not a monk by that time) openly criticized the extremely powerful church and questioned the actions and authority of its leaders. The theses were not viewed as harmless by them. There was no freedom of religious expression. People were sometimes executed for heresy by both Catholic and, later, Protestant monarchs- in England by (amongst others) Henry VIII (Protestant), his daughter Mary I (Catholic) and later her sister Elizabeth I (Protestant), which must have been confusing for the people.
I don't think the analogy of the Reformation, and subsequent religious strife, with a scenario in which a UFO is demonstrated to likely be an alien craft is very strong.
What people believe and say about UFOs is a matter of personal choice. Most of us, and our respective societies, are far more tolerant than the authorities were in the 16th century. There is no persecution of "believers", as numerous conferences, websites, shelves in bookshops, TV shows etc. etc. demonstrate. Police in several countries have been amenable to investigating UFO reports; they don't treat claimants as suspects or dissidents.
In recent history, in our modern nations people have had to adapt to new, deeply troubling information. I've mentioned elsewhere (also
@Todd Feinman, post #134) the Cold War. Most informed adults in many nations would have been aware that their societies could be destroyed, and their lives ended or horrifically impacted with just a few minute's notice. It wasn't a theoretical threat; the means to accomplish this existed, were targeted and actively maintained. But faced with this existential threat, the vast majority of people led their lives as they would otherwise have done. I doubt proof of the reality of ETI craft could be
more frightening or socially disruptive.
Most scientists involved in astronomy, physics etc. probably don't believe that UFOs represent alien craft. A minority do. This is probably because testable evidence is lacking (and we have no testable, convincing evidence for any form of extraterrestrial life, let alone ET intelligence, yet). If objectively convincing evidence existed, presumably many (e.g.) astronomers would change their views (although as in other scientific debates, no doubt some would have different views on what constituted firm evidence and might maintain minority viewpoints).
Opinions about UFOs amongst most relevant scientists isn't (I hope) about belief, or supporting an orthodoxy, or what they might want to be true, it is about what the evidence suggests.
There aren't any laws of physics that rule out the existence of ETI or interstellar travel. Evidence of ETI visiting Earth would not, scientifically speaking, be a paradigm shift (though perhaps it would be surprising to many).
Many people in developed countries believe that UFOs might be alien craft
anyway (polls vary, perhaps 30-40% of people when polled on the subject). Beliefs about UFOs don't appear (to me) to be contentious in the same way that beliefs about, e.g., immigration, terminations of pregnancy or perhaps gun ownership rights are in the USA, or the arguments about the UK leaving the European Union, or disagreements in France whenever the government attempts to limit social spending / pensions/ subsidies.
The situation is very explosive...
I don't think it is. No mass demonstrations,, no clashes with police in riot gear, no acts of violence by extremist lunatics. No tractors blocking highways or fuel depots. No "believers" taken in for questioning or storming the Pentagon. No-one expelled from astronomy or physics degrees for believing UFOs might be alien craft. Opinion polls ranking matters of concern to the public are not uncommon; I don't recall any, in any nation, mentioning UFOs.
