You can do a "custom format cells" that will display the opposite sign. Select the cells involved, right-click Format Cells, Number tab, click "Custom", and then in the small box just under the word "Type" on the right side, enter this:
(0.00)_);0.00
Basically, I initially picked a number format that had 2 decimals and then went into custom format and added the () on the left of the semicolon and removed the () from the right of the semicolon. So you can use this technique to select your desired display format 1st and then in "Custom" reverse the () or the - if you prefer the minus sign. The cell value does not change, only the displayed amount has the opposite sign.