If you create a calculated field that gives you a discrete result for up/down/no change, then you can put that field on the shape shelf and assign a different shape for each result.
The code for the calculated field would look something like:
IF [Calculation1] > 0 THEN "Up"
ELSEIF [Calculation1] = 0 THEN "No Change"
ELSEIF [Calculation1] < 0 THEN "Down"
Then you would give "Up" and up arrow, "Down" a down arrow, and some other shape for "No Change".