New information and research comes to light all the time. Especially informative are dive or ROV explorations that find boats "where they shouldn't be". There is also a process where claimed sinkings can be assessed against reported attacks -- in these cases, if the boat lived to report it, the boat wasn't sunk.

And I think that Dr. Neistle' will be the first to agree that in many cases it's just "best guesswork". You take a bunch of disparate data (some of which may be incorrect) and try to make sense of it. It sometimes only takes one reliable or authenticated fact to make an earlier conclusion obviously incorrect.

Besides this, there are the disappearanes of boats that can't be explained with certainty. You may have three Liberator attacks south of Iceland on the same day when there were only two boats within 500 miles of that spot; the Liberator crews are absolutely certain that they each sank a boat -- and one of the boats returns to port without reporting an attack on that date. Who sank the missing boat? Was it multiple attacks on the same boat? Did the boat go missing as a result of some cause not even related to the reported attacks?

And there's "big picture" effects. Especially after the closure of the French bases after D-Day, the British put a big effort into minefields as an effort to bottle up boats from north-German or Norwegian bases or restrict their movements. And in many cases, mines were silent U-boat killers -- the boat simply sailed away and was never heard of again. But in the last few years, underwater exploration has located some boats and has shown that the minefields were more effective than previously thought. A good example of this is the latest information on the minefields off western Cornwall. See [www.uboat.net]

And it's helpful to reflect on the wartime situations. Interrogation information from captured U-boat sailors was of great value and much sought after during the war; also, analysis of attacks and the best guess of effectiveness was valuable information to reflect on the validity of tactics and operations. If the Allies knew where U-boats were sunk and what killed them, they could fine tune efforts against them. So, many fates were quickly assessed by the Allies with little information. Late in the war, such information becase less valuable and suddenly (at the end of the war) it became only interesting as historical information. There was also some feeling that Allies sailors should be rewarded - if a boat reported data that seemed likely to indicate that it sunk a U-boat, there was pressure to "award" a kill to the boat, congratulate the crew, and make what propaganda use as appropriate. Later information might conclusively prove that the reported sinking did not occur. In such cases, you have to look at other attacks and lost boats and reassess previously reported fates.

Recent research is also pointing toward the obvious fact that submarine life in WWII was simply dangerous. There are instances of boats being in a Norwegian port for repairs. The boat would said but would return in hours reporting that a test dive has shown dangerous equipment malfunctions. After more repairs, the boat would sail again and quickly return. After even more repairs, the boat would depart yet again and never be heard of. But two weeks later, an Allied aircraft or vessel would report dropping depth charges on a "possible" contact right where the boat should have been on it's planned patrol course. In earlier times, the claimed sinking would have likely been awarded to the claimants but maybe cooler heads from the vantage point of a few years wouldn't be so sure. (On the other hand, there were a number of instances where work was completed on boats and the boat would sail specfically on a test voyage with a plan to test dive and return within hours and never reappear; in such cases, it's highly likely that the boat sank because of a mechanical failure.)

So, there has been ongoing reassessment of fates. Sometimes an admittedly shaky conclusion can be easily disproved. Other times, an earlier assessment can be shown to be false but any other explanation seems only speculative.

There is no one answer to your question, but in a bigger sense it seems clear that fates often can and should be reassessed. But it's a big ocean out there and it keeps many secrets. Some fates will be confidently reassessed in the future; others, we'll get more data but no real proof, and a final group we'll just never know.