Changing pick-up bar

I’d like to change the pick-up bar in the attached example to be a full quarter note in length rather than an 1/8th so that the timpani part can be a triplet that’s a 1/4 note in length rather than an eighth in length.

If I double-click on the existing time signature signpost and change the pick up from .5 to 1, all the music afterwards gets shifted by an 1/8th.

How do I add an 1/8th to the pick-up bar or what other strategy might I use?

Try putting the caret at the beginning and using the Shift+B popover to insert one eighth-worth of time by typing +1/8 into the popover. Then do 2/4,1 in the Shift+M popover to replace the initial time signature and you should be all set.