How do I change a rotation point mid tween?

I am very new to Animate so don't know the full terminology for what I'm asking for yet, so sorry if this sounds confusing.

I have a simple rectangle. I have applied a motion tween and within that I have then moved the rectangle around and also spun it on the pivot point which is currently in the middle of the rectangle. This is what I want it to do.

I then want to move the rectangle some more and rotate it again, this time around one of the corners instead.

However, whenever I move the rotation point (the white circle) to the corner I now want to pivot around, it also changes my previous rotation. I don't want to change the previous one, only the new one.

Does anyone have any idea how I overcome this?

Thanks in advance.