I do not get what are the supposed changes on that aircraft. You would think this would be tests of the Su-57M chassis, was supposed to come out this year and feature all electric actuators to replace existing hydraulics, but they put a picture of T-50-11 on the press release. So maybe they just changed some of the electronic systems on the aircraft and are testing those? Another option is that the picture is not representative of the aircraft being tested.
"Russia’s upgraded Sukhoi Su-57 fifth-generation fighter jet performed its debut flight on October 21
...
The Su-57 fighter jet is also testing onboard equipment with extended functionality, the crew’s AI support and the option of using a broad range of new types of weapons, the UAC said.
"The aircraft also offers a possibility to install a second-stage engine," the statement reads."