Probably the most obvious change: sliders are replaced by drop-down menus to allow parts and wearables to be selected more easily by name (and soon ordered alphabetically). The change works equally well on Windows and Android. Of course, sliders remain the preferred control for the face / body morphs.
Under the hood there is a huge change with regards to how the character save files look - if before a body part or an item were saved by their index in various internal lists, now they are each saved by a newly introduced unique ID. This simplifies the code (making for easier maintenance and development), is a lot more flexible and reliable long term, and also allows for items to be renamed and lists to be sorted (as mentioned, they will become alphabetical, to make it easier to find things). This also allows e.g. removing an item from an internal list without messing up all subsequent indices, and therefore previously saved characters.
It's also the first step towards creating a character sharing feature (i.e. exporting/importing characters, and sharing the files between players). Sharing will be a lot easier to add and work reliably, even between multiple (future) versions.
At the same time, backwards compatibility is preserved (even with the old version with no material support). You will not be not losing any existing character customizations.