Changes actually make plenty of sense, just not really sure I feel they are the right changes.
Effectively the just nerfed her CDs so her Q and E are both longer, and frankly these are pretty big. Janna maxes E, which is 13 second CD at all ranks now meaning when she has 45% CDR the spell will have a CD of around 7-8 seconds up from the 6 second CD with 5 second duration shield she has now. They also nerfed the CD on her Q and considering she maxes that last this is a bloody huge nerf to her ability to peel in the early game.
The big change though comes from her W, which now refunds mana and reduced her CDs by 2 seconds when she casts it on enemies. This is a pretty low range ability and reduces her move speed when doing so. With max CD you are looking at a 6.5 CD on it, and it reduced her other CDs.
In the early and mid game this really hurts Janna's peeling before she gets all her CDR. 12 second CD to reduce your other CDs by 2 seconds while the Q CD will be a 17 second CD fucking hurts like hell when you are trying to peel champs with a million different gap closers. Late game it makes her a peeling machine if she can position well to spam W, assuming it reduces it's own CD putting it to around a 4 second CD giving you nice reductions on your other spells.
Time will tell but they are hitting her early and mid game peeling to give her late game god levels of peeling IF she puts herself in harms way to cast her W on someone. Frankly I think the whole nerf is pretty dumb and I see zero problem with Janna's E as is. If Janna has a problem, which I personally feel is debatable, it's that a good Janna right now will make it next to impossible for you to touch her carries and if you do she will just blow you away and reset the fight with her ult.
They are trying to hit that with these nerfs, but I don't feel this is the right way to do it.