My Mom had a total knee replacement in December 2017. Her first trip back to WDW was not until September 2018, but she started back with Disneyland visits in February 2018 (we live local and only went a few hours). I definitely suggest the wheelchair or ECV as the walking and standing was still too much for her (and honestly still is, but she should probably get the other knee done too). The first few times at only 2 months post TKR, we did rides that she did not have to step up or down into. But then we started to add those in and have been fine. She find the back seat for Test Track / RSR easier for her as she has something better to use to help her stand up getting off. She has no problems with FoP or Pirates at all. She does not ride EE, but would assume that would be ok to get in and out of. Also note, there are some rides like Big Thunder (well at DL at least) that they can drop the side down on the car to make it easier for the transfer, so you can always ask about what you need. The CMs will ask about the transferring and if you think more time will be needed and those sorts of questions.