This whole Sonic debacle is actually producing some interesting things about people explaining how they can change the Sonic model and how difficult it will be to do so...
A VR presentation... better than any powerpoint lol.
That's a nice video with a pretty good quick and dirty overview of the process, and I love the VR format as well. He's mostly right but does sort of gloss over some of the difficulties and finer details (which is understandable given the length of the video but a bit misleading.)
They'll of course have to actually redesign sonic which requires new concepting, modeling iterations, new materials, and rendering tests. There have been some great fan concepts but actually getting the finished render to look like the concept can be challenging.
Once the model is redesigned it'll likely be different enough that the rigging will need to be redone. For those unfamiliar, rigging is the process of assigning each part of the 3D model to a skeletal system that holds all of the animation data while making sure everything bends and deforms correctly. This is generally a pretty tedious and technical process. If the proportions are different enough that the skeleton itself needs editing as well it may break the mocap data which would need to be reprocessed.
The bulk of the work will be in animation, particularly the facial animation. Facial mocap is really only good for a rough estimation of the actor's performance and most of what you see in the final result is actually manually animated (usually using footage of the actor's performance as reference.) It's likely the vast majority of the facial rigging and animation will need to be redone.
Much of the texture data may be unusable because the UV map of the new model will be different from the original model. A UV map is a coordinate system that tells the renderer how to map the flat texture images around the surface of the model. As mentioned in the video, textures are used in a lot of different ways to define attributes on the model (color, shininess, translucency, etc.), so each of those attributes will need a new texture specific to the new model's UV map.
They'll be able to use some of the existing values for the appearance and material of the fur, but it'll need to be re-groomed, which is its own specific modeling process where you define the length and direction of the fur. This can be achieved through a combination of attribute textures and special grooming tools (yes, virtual combs are a thing.) The fur physics will need to be resimulated so that it responds to Sonic's movements and to outside forces like wind.
Other physics systems will need to be resimulated as well if they interact with Sonic, like water, smoke, and cloth. These systems will already be set up so they should just be able to rerun the simulations with the new model, but it's still time consuming.
He covered compositing pretty well and much of that existing work should be usable, but if an actor interacts with Sonic--for example grabbing his arm or giving him a hug--that's particularly difficult and generally requires reproducing that actor's movements in 3D (which they'll need to update to line up with the new model) as well as extensive rotoscoping (manually airbrushing frame-by-frame.)
Overall it's a pretty tedious and complicated process, so it's good they pushed the release date back some to give the team more time.