Look, if you need to change the series that dramatically for it to work in a TV format, it doesn't work in a TV format.
F'serious, yo. For an Animorphs series to even be considered viable it needs something (impossible) like a James Cameron/Steven Spielberg collaboration on a $300 mil budget for the bigger movies. Maybe have K.A. and Michael with some executive producer influence, and a chief producer who respects the material. And don't let Jim Cameron near the script, limit his power to the lensing and set imagination. And don't let Spielberg bring his buddy Lucas within a 500 mile radius.
It's all very much intimidating, and basically unfilmable due to the ageing of the kid actors. TV would solve that, but TV wouldn't have a tenth of th budget required to make people buy the scope of this, or detail the effects-heavy sequences.
Oh, and no screen adaptation, large or small, can ever top the imagination of a kid in the book's target audience. Better left untarnished.