It's a big of a workaround, but you might try Custom Animation and triggers.
To do so:
1. Go to the Animations tab, in the Animations group, and open the Custom
Animation task pane.
2. Select your command button.
3. In the Custom Animation task pane, click Add Effect, and select an Exit
effect. I recommend "Disappear."
4. Select the Exit effect in the Custom Animation task pane list. Point to
the arrow on the right side of the effect, and select Timing.
5. On the Timing tab, select Triggers. In the drop-down menu, select "On
Click" of your command button.
6. Copy and paste your command button. Edit the text to read "500."
7. Select your second command button on the slide and place it so that it
completely overlaps your first command button. (It should "cover up" the
first.)
8. In the Custom Animation task pane list, click Add Effect, and select an
Entrance effect. I would try "Appear."
9. Select this Entrance effect, point to Timing, and on the Timing tab, set
the trigger so that it is "On Click" of your first command button.
10. With that second command button select, in the Custom Animation task
pane, click Add Effect, and select another Exit effect ("Disappear").
12. Select this Exit effect in the Custom Animation task pane, select
Timing, and on the Timing tab, set the trigger so that it is "On Click" of
your second command button.
I hope that works for you!