It's usually a pretty easy conversion from 27.5 to 700c wheels, but the fact that your 520 has cantilever brakes might be an issue. If you do stick with 27.5 you'd probably either have to build a wheel with a dyno hub yourself or find someone to do it for you. It's not impossible.

Otherwise I don't see any particular reason not to keep your 520. They are highly regarding touring bikes. You might be sorry you sold it. With some upgrades it would be as enjoyable to ride if not more so than the other bikes you're thinking of.

On the other hand, they do fetch some pretty good money. If you sold it, it wouldn't be that much more expensive to get a new bike with the components you like compared to upgrading everything on the 520.

If it were me, unless there was something in particular about the geometry I didn't like, I'd probably keep it.