Riku didn't resist till Ansem tried to hurt Kairi. Whether it contradicts or not wasn't a concern of mine either since I was speaking from a development angle. I was pointing out I just attribute it to information added later after Kh1 existed. During KH1 nothing past KH2 ideas was thought of so naturally the difference shows.
If your looking for a in-universe answer your probably not going to find one outside guesses.
I honestly think that as soon as ASOD took Riku's body, he would have started to resist. I don't think he realised he was going to be possessed, have his body ripped from his control...we just didn't or haven't seen/heard of that initial resistance yet - I don't think Riku would have been that pleased, even if he did submit willingly... He was quite naive. Threatening Kairi was just the last straw and it gave Ansem the excuse to rip Riku's heart out of his own body and send it to the Realm of Darkness.
As for the question of this thread... I think Ansem wanted to freak Sora out by allowing him to see his friend in this way, being completely evil. Ansem only shaped Riku's body to the form we see him in after the Hollow Bastion fight. So he did change the body he had possessed but only after Sora had freed Kairi's heart.
Though, thinking about this topic has made me wonder how Riku got his body back... Wouldn't it have been destroyed when Sora defeated Ansem? How does that work when it comes to possessing another persons body?