Riot's plan with the Yuumi changes is to give her more incentive to unattach and put herself into more vulnerable situations, however the changes on the PBE are actually having the opposite effect.
Riot is trying to give Yuumi players more incentive to unattach by giving her passive a slightly bigger shield, Mana gain on the empowered auto attack, and increased Mana costs. By doing this, Riot is telling the players to unattach and put themselves into a vulnerable situation that will give her kit more counterplay. In theory these changes work, however with the changes to her W, they are further telling her to stay attached. Increased Mana costs just means that Yuumi will just grab Presence Of Mind along with mana flow band. These two runes alone make the increased mana costs almost useless, and the amount of Mana Regen Yuumi naturally builds means giving her Mana back on her autos useless outside of the very early game.
The W changes that will put her W on a 5 second cool down further tells the Yuumi player to never take risks in fear of dieing, this will make her stay attached for a longer period of time, making her have less counterplay. The channel on her W will do the same thing, as it will make the Yuumi player less likely to unattach. Not to mention, her W adaptive force trade got buffed, meaning she is now being told to stay attached.
With only 1 charge of her E, the Yuumi player is being told to not open themselves to retaliation by trying to jump from ally to ally during a team fight.
If riot wanted to have the Yuumi player's unattach more often and out themselves into harm's way, some more appropriate changes would be as follows:
• Decreased shied strength, but lower cool down on the shield
• Increased Q cool down (let's say, 22 seconds rank 1), that gets lower if you auto attack an enemy champion
• Lower E healing unless you currently have your shield on you/an ally, in which case the healing is stronger
• Move the slow off of the empowered Q and onto her passive.