expand or shrink one measure

When I'm composing, I sometimes want to change the length of one measure. Is there a way to add or subtract a beat to a measure without it affecting the other measures around it? I can insert a measure, but I don't see an option of what length of measure to add. It just adds whatever measure is selected (I think).
So, for example, if I wanted to change a 6/8 measure with lyric, notes and chord symbols to 4/4 in the middle of a piece, what would be the most efficient way to do this? As it is now, I have to insert a measure, cut, paste, change time signatures until I get one measure with nothing in it that I can cut out. It gets confusing if I don't put a time signature in a measure soon after the one to be altered because everything shifts.