They should do a re-release rather than an entire rewrite of the original series. If anything would be changed, maybe make sure there's more continuity. Also, maybe have K.A. re-write the lesser quality books written by some of the ghostwriters.
Or if it's easier, maybe release summary books including an overview of important events and storylines.
And then start a series of new books picking up from book 54, or some alternate version of book 54.
As for what age group the books should be aimed at... I always felt as if the Animorphs books could be enjoyed by anyone. But financially speaking for the series to be successful it should be aimed at young adults as they were before, but written in the same style as the originals. Because that's what we still enjoy about them eight years later, right?
Scholastic should NOT try to emulate Twilight, Harry Potter, etc. Animorphs is too good to stoop to that level. Give KA management of the project and don't have any more ghostwriters than is necessary to continue the series on a regular schedule.