Earlier this year, the Major League Baseball team the Philadelphia Phillies revealed a new look version of their mascot the Phanatic (Twitter link). The revisions include: a set of flightless feathers, bushier eyebrows, solid blue tail a larger butt, a different shade of green fur, hands are fur free, blue and white socks, and red shoes.
This new look reportedly is due to a declaratory judgment copyright lawsuit (The Phillies v. Harrison/Erickson, Inc., S.D.N.Y. Case 1:19-cv-07239, Aug 2, 2019, Complaint and Answer) initiated by the baseball team against Wayde Harrison and Bonnie Erickson, the creators of the mascot.
Harrison and Erickson created the mascot in 1978 and they assigned their rights to the the mascot to the Philadelphia Phillies. In 1984, Harrison and Erickson renegotiated their agreement and assigned all of their rights "forever". However, in 2018, Harrison and Erickson gave the Phillies notice of their intent to terminate their original copyright grant to the Phillies under U.S. Copyright law, 17 U.S.C. 203(a)(3). In response to the threat of termination, the Phillies sued the creators and revised their mascot.
The heart of the matter seems to be whether the 1984 copyright holder's assignment of all their rights "forever" overrides the copyright law which states that "termination of [a copyright grant] may be effected notwithstanding any agreement to the contrary" 17 USC 203(a)(5).
If the creators win, this new look -- which involves minor changes to the original character (see below image) -- probably is an unlawful derivative work.
Image from Twitter used under fair use.
Image of original character from Mobilus in Mobili.