Mostly it’d just be about dusting off the cobwebs and using it all again; I still know it all, but my skill will doubtless have atrophied at least a little from six or seven years of disuse. In such a case, having been familiar with Python 2.5–2.7 first probably also works against me, as the distance of time will probably make recollecting the ancient Python 2 way occur more often relative to the Python 3 way than it would have while I was actively using Python 3. As far as actual ecosystem changes are concerned, packaging practices are probably the most significant area where I gather there’s been a fair bit of change happening.