I read that Ars Technica article just before I posted my question. I've been programming since the late 1960s, so I know languages change. What I'm hoping to do is pick up Swift at a point where changes are more incremental than fundamental. There were big changes from 2 to 3 (from what I can tell), but it looks like mostly behind-the-scenes changes in store for 3 to 4 (like stabilizing the ABI and the .swiftmodule binary file format). You can't hold off forever (like waiting for the perfect Mac Mini), but you can try to avoid learning a language when it's about to have large chunks of it deprecated or dropped in the next version.
It sounds like you can use your experience as a guide.
I would start by going through the list of deprecations and changes in Swift 3 relative to Swift 2. If it seems like a lot of really fundamental stuff you'd have to "unlearn", then that suggests using books and tutorials targeting only Swift 3. You'd then have to account for the availability or absence of learning materials that target only Swift 3. You'd also have to account for the target audience of the learning materials: it may be that Swift 3 materials are largely targeted at people who know Swift 2, and would be confusing to those with no Swift knowledge at all.
As an example, for myself, I wouldn't much care about reduction in functionality of the 'for' loop, because that doesn't strike me as fundamental enough. Yeah, it matters a lot if you're trying to migrate existing code that relies on the removed capability, because that's a rip-and-replace kind of change. But in terms of learning the language, loops is loops, and I've seen enough of them in Basic, Forth, Fortran (implied DO (yay)), Java (iterators and non-iterators), etc. that the feature-set of a 'for' loop isn't going to be a huge obstacle. That's my assessment, as it applies to me, from my experience. YMMV.
If there's some particular thing in Swift 3 that you're not sure about, then please ask about that particular thing.
On a side note, it's good that you mentioned your past experience, though if it had been in the 1st post it would have given an important context for the question. When I see a question that doesn't list past experience, I start by assuming it's from someone with no programming experience at all.
