Was there a point in the game where "Sephiroth" changed HIS appearance?
Sephiroth was a normal looking human five years ago. Then when you see him again in the Northern Crater he's mostly human but his legs are all tentacle Kishi style. At the end he totally morphs into the monster you fight. Sure, he's the last boss, but it's easy enough to follow that he morphed using the Jenny cells in him.

Knowing that informing Cloud of his unfortunate experiences at SOLDIER would turn him, "Sephiroth" lied to Cloud about clones being able to change form. Actually, the "clones" are just regular people from SOLDIER who were injected with Jenova Cells.

Jenova probably used this deception to cause every other clone to go insane and search for the "Promised Land" as well.
Jenova wasn't just calling Cloud, she was calling all the injected. Your forgetting one thing though. It Jenny was free to use different limbs to do all these different things, then why didn't she just free herself and go after those injected with her cells. She needed them to become free. She was most easily able to seduce Sephiroth, making him want to fuse with his "Mother", because he was so infused with the cells. Sephiroth was her main concern. She needed to call the injected to him to help him so he would be ale to free her. They morph into pieces of Jenova through their Cell infusions.