As it stands there are 4 main categories of question that need to be considered:
- Questions relating to the new canon that are fully answered with reference to the new canon only.
- Questions relating to the old EU that are fully answered with reference to the old EU only.
- Questions relating to the new canon that are answered by pulling in information from the old EU.
- Questions relating to the old EU that are answered by pulling in information from the new canon.
Of these the only one that is potentially troublesome is number 3. And I say "potentially" because an element from the old EU may yet be pulled into the new canon which would have the effect of making the answer valid.
Number 1 is evidently self-contained and need not be touched.
Number 2 is also self-contained; it doesn't cross a canon-boundary so there's no need to do anything about it.
Number 4 is also OK: since the new canon takes priority over the old EU, an answer referencing the new canon must always be valid.
Regarding number 3, in reality (and from a purely pragmatic perspective) little has actually changed. Under the old hierarchical canon, something in what was formerly "G-canon" or "T-canon" would always trump EU material, and it was never the case that anything at this level would have had to be beholden to EU material. A "G-canon" or "T-canon" work could have blatantly contradicted an EU work, and that would have just been tough cheese.
It was also always the case that the EU was always viewed as something that may or may not have happened, and should never have been read as something that definitely did happen:
The analogy is that every piece of published Star Wars fiction is a window into the 'real' Star Wars universe. Some windows are a bit foggier than others. Some are decidedly abstract. But each contains a nugget of truth to them. Like the great Jedi Knight Obi-Wan Kenobi said, 'many of the truths we cling to depend greatly on our own point of view.'
I don't read that stuff. I haven't read any of the novels. I don't know anything about that world. That's a different world than my world. But I do try to keep it consistent. The way I do it now is they have a Star Wars Encyclopedia. So if I come up with a name or something else, I look it up and see if it has already been used. When I said [other people] could make their own Star Wars stories, we decided that, like Star Trek, we would have two universes: My universe and then this other one. They try to make their universe as consistent with mine as possible, but obviously they get enthusiastic and want to go off in other directions.
Legends merely formalizes this, and the new movies are just doing what they have always had the right to do: supersede EU material. Even if the old EU had never been de-canonized, even under the old system, the scriptwriters of the sequel trilogy have never had an obligation to use any or all of the extant EU material. This was always a risk when relying on EU material when sourcing an answer.
The question is not so much "How should we handle Star Wars Q&As post-Disney-canon-revamp?" but rather "How should we handle potential contradictions between the (old) EU and the upcoming sequel trilogy?", and that would have always been the case, even if there had not been a canon-revamp.
The best approach therefore seems to be to do nothing for now. The answer is still valid as of the time that the question was asked, so it's served it's purpose. Stack Exchange is a Q&A platform, not a repository of canon. Unless material in an answer is explicitly contradicted by new canon, there is no reason to change it. If material in an answer is never contradicted by new canon, there is never a reason to change it. Otherwise it can be handled on a case-by-case basis.