Can Showstopper target and move a unit that is already buffed?

Ruling: Yes, you can target an already buffed unit with Showstopper. When it resolves, you do as much as you can - you won't buff it (unless it's Lee Sin), but you will still move it.

Sequence:

• Target a friendly unit in your base (buffed or unbuffed)
• When Showstopper resolves, apply buff if possible
• Move the unit

Nuances:

• Lee Sin is an exception that can be buffed even if already buffed