Does deploying flap change the aspect ratio of the wing?
The aspect ratio of a wing is defined as:

$$ \text{AR} \equiv \frac{s^2}{A} , $$

where $s$ is the wingspan and $A$ is the total wing area.

Some types of flaps are not only increasing wing camber, but also the wing area:


  The Fowler, Fairey-Youngman and Gouge types of flap increase the wing area in addition to changing the camber.


(Wikipedia)

Since the wingspan is unchanged, this will decrease the aspect ratio. Other types of flaps, like a plain flap, are not changing the aspect ratio.

You can see the flap types that change the aspect ratio in the following image (modified from Wikimedia):