As excited as I am to see and use some of the new versions of classes, I'd probably still be inclined to using older versions occasionally. Has it been stated how choosing classes in dndbeyond is going to be? Like, are we going to have just two of every class except for Artificer and Blood Hunter? 

Also, separate but important question, with how classes are changing to all have their subclasses at level 3, how are introduced subclasses going to interact with the new versions, and on the same note, are the new subclasses backwards compatible with the old versions of the classes?

We won’t really know until they come out. We have been told that they will be backward compatible to at least Tasha. I am assuming (a risk I know) that subclasses in the new PHB will supersede the old ones and that the old ones may not be compatible. For everything not covered in the new PHB I’m assuming that most subclasses will still be compatible with a few possible exceptions like the hexblade where many of its L1 features may have been subsumed into the general class features.

An interesting question will be if the character generator is ready on release and how they are going to integrate with the old characters you already have?