Do you mean change them to half REST and quarter REST?

Select the passage in question, choose Utilities>Rebar Music. This will work in 4/4, but might give you some unexpected results if you are in a composed time signature like 7/4.

If you are in an unusual time signature, the best way might be to change one manually and use Jari Williamson's JW Rhythm Copy plugin to copy it to the other measures. In fact, if you have no pitches to change, simply change one, select JUST THE RESTS in the partial measure carefully and alt-click it into the other measures. Once you have a whole measure stack the way you like it, you can copy it in one gesture into the next measure this way.