Why this is not sanae?
Stars in background, and kazami yuka's eyes are red.

Kazami Yuuka's eyes were green back in PC-98.

And she used pants instead of a skirt back then...

So why/how is she topless anyway?

We can only explain away so much by saying "It's PC-98," okay? Let's just enjoy the toplessness while its there.

Alignn said:
So why/how is she topless anyway?

This is stage six, and Reimu got to Yuuka before she could finish changing clothes.

angrybull said:
This is stage six, and Reimu got to Yuuka before she could finish changing clothes.

This is the most likely option.
To make things more clear. The heroine (Reimu in this case) gets to wake up Yuka at stage 5 of Lotus Land Story, where she appears in pajamas. After a battle, Yuuka flees and reappears as a final boss, dressed in plaid vest and pants.

Oh wow, this is the first I noticed that classic Yuuka had long hair.