The real nerf Janna needs to her E
Current PBE changes just makes her more passive in lane because she'll just wait for her E to be up again in order to make a safe trade.
What she really needs is for her shield duration to not be 5 seconds at all ranks.
5 seconds during the early game not only means she wins out on immediate shielding but any extended trades due to this, shields are meant to be patient thing to play around but after basing and getting coin upgrade she'll end up with about 10% CDR which means she'll have about 3 second delay between her next shield and the duration. (Live CD values)
If you end up keeping the current values with nerfing her shield duration instead of giving a hefty cool down this would actually be better to punish Janna players for incorrect shielding, giving her a higher cool down just promotes more passiveness.
A good duration would probably be 2.5/3/3.5/4/4.5.
This will still keep Janna late game security of being able to protect her carries but open up more early game punishment for incorrect shielding and extended trades.
If the current changes to her E go live, it's not going to accomplish much.