Why does it matter if they change their minds as to the format of the release?
Because they'll be lying to the audience. Like hard core lying. Most animated films typically take 2-3 years to make. While home video takes one year to make, give or take. You are quite obviously going to make a much, much better animated film over the 2-3 years (and this is for ALL animated films - including non-Pixar animated films).
So you have them lying to the audience in terms of quality straight from the start. If they truly did believe in it and saw that it could stand on its own two feet - it would have been slated as a theatrical film from the start. That it's being quite obviously shifted through theatrical shows that this is just a money-grabbing scheme with not as much thought put into it on the creative side.
As said, if they tell the audience the truth of the matter - alright, they're not robbing them. If they tote this off as another disney animated film they've been working on and have just been keeping quiet about - that's a whole other thing all together. There's releasing home video to theaters and then there's tricking the audience into believing it was always meant for theatrical. One is blatantly lying to the audience, the other is letting the audience make up their minds with the right information provided to them.
So why does it matter?
1) If they truly had faith in it, it would have been theatrical to begin with. This tells a lot about the script right from the start. This also tells us a lot about the amount of work that went into it in comparison.
2) Due to the above it screams cash grab and manipulating the audience into think it's something that it's not. The masses, if Disney does what I think they will, will think that it's just another Pixar film and have absolutely no clue that it was originally meant for home video. It shows pure financial over creative motivations.
3) I do not believe in lying to the audience.