You are in LUCK!

That ESC ( I have one of these too ) is LiPo compatible and it will accept 11v input.

Look closely at it.

There should be a jumper. It may or may not be labeled.

Put a mark next to how it is now, then move the Jumper to the other two pins.

After doing that it will be able to handle LiPo batteries.

It's the same ESC used in several different brushed planes.

The warning about the number of cells sort of does not apply here as you will be installing a smaller battery with higher capacity and voltage.