I don't think they will. David Karp is signed on with the tumblr staff for four years and Yahoo is going to pretty much let it stay the same, operating independently. Yahoo doesn't have the same owner anymore, so I'm not going to treat Marissa as the same person responsible for things Yahoo decided to do in the past. With experience from Google under her belt, I'm pretty sure she knows better than to do something that will just drive away exactly the audience she wants to collect.
So far from what I've noticed, there seems to be a growing trend of leaving the E out of more product names. I'm hoping with the money from Yahoo, things users have been wanting changed or fixed will finally get done, like tumblr mobile, embedded videos and maybe even apply some things that Missing e and Xkit offer. I'd especially like to see something added to allow you to set a post as followers/following-only, disable reblogging, and uploading images in the body text of a music post, and removing the default setting to reblog posts as links.
I saw somewhere that if Yahoo hadn't bought Tumblr, then Tumblr only had enough money to keep running for about three more months before having to shut down.