If the US is moved from May, June or July 1917 to a September 1914 world and seasonally appropriate weather, that can cause a drastic shortening of the United States 1917 growing season, grain crop, and harvest, a big problem for the Americans and whole Entente.
Good point.
@stevep - you mention the Ottomans, or in your estimate the Bulgarians, being deterred from joining in. Especially from receiving a mid-1917 vantage point, might downtime Italy and Romania be deterred from joining in? But at the same time future knowledge makes the CP think about perhaps preempting.
Also a good point. They could try attacking Italy or Romania early but that's likely to backfire. However until the 1915 victories in Serbia and Russian Poland how much spare capacity in terms of troops, supplies etc they have. Also an early attack on Romania would not only stretch the CPs further but without Bulgaria already in the CP and German forces based there plus the Russian army still in decent - for it anyway - condition that could be a tough fight. Or do you mean something different. Plus while the 1917 US knows what happens its still not happened in the 1914 world so despite what their hearing from the US it would still seem like naked aggression by the CPs which would unify their opponents.
With Turkey I would expect it to still join the CPs as the ruling clique are already heavily committed but they might get strong pressure, including possibly from Berlin, not to commit widespread slaughter of minorities. Bulgaria with its dispute with Serbia could even move earlier but that would leave them exposed if Serbia gets aid. An interesting situation would be what happens in Greece where effectively the allies organised pretty much a coup backed by the Prime Minister against the king who was pro-German/hardline neutral depending on what source you read.
Overall I think the EPs will do better with 1917 information because they will have markedly better access to it. Both because the US is by this time distinctly hostile to the CPs and because EP people have more access to the US itself including a lot of people in the country. There might be some German-American groups who may want to help the CPs but their going to have difficulty getting info to the beleaguered powers.
Agreed. However, I think in answer to the OP, the Germans are going to be too irrevocably committed to the U-Boat weapon to drop it, even after learning of the change in America. The strong British blockade and withering British assaults in Flanders will convince the Germans they cannot abandon the U-Boat weapon and that they just have to play it out and win with it before the Americans can intervene in the war effectively. Possibly, if the Germans continue with it through the Bolshevik October revolution, and downtime America has not declared war yet, Germany might offer to suspend or halt attacks on neutral shipping if America stays neutral. - But it will be awful hard for the USA to trust Germany's word by this point and tolerate that many months of ship sinkings. And it will be tempting for the Germans to see if they can redeploy forces from the east for a victory in the west before America can send over relevant numbers of trained and equipped men to the western front.
One factor here might be US people including sailors and military people outside the US at the point of the ISOT. Especially anyone who has been on a ship, US flagged or otherwise sunk by U boats. Its going to complicate matters because many of them will have their 1914 equivalents now in the US so there's going to be a lot of personal and legal strife resulting there. Not to mention its going to have an impact with 1914 people who get told "in 1915/16/17 you got killed by a U boat attack".
However the EPs will be hit by the fact that the US is now - at least in its eyes - at peace and also that it hasn't gotten the development and knowledge from 3 years of allied purchases.
One issue here is that since Germany is at war with the US there's likely to be some US ships sunk before Berlin, let alone its sub commanders realise that something has changed. This could change opinion fairly quickly in the US.
Does the ISOT in either case affect US people/forces outside the continental US [and possibly Hawaii]? If not then there's going to be people living overseas and in the 1914 to 1917 world probably a fair amount of military personnel who are aware their at war with Germany and could further complicate matters. Not sure when the USN started supporting ASW efforts in numbers or sent a battle squadron to Scapa to join the Grand Fleet. In the 1917 to 1914 the people overseas are likely to be less in number because you will have less military abroad. However still potentially forces at sea or in the Philippines and the like.