Well, it kind of changes the API. But the most common code path here has always returned querysets, so callers are likely prepared to handle that (like both callers in this file). How good is "likely" and whether they handle it correctly is another question, but if not, their code was likely broken anyway.

So it's a judgement call, I would rather leave that up to CMS developers, you guys have a better understanding of the customer base than I do. Perhaps it's reasonable to simply document this as an API change in 3.1 and be done with it?