What unit/situation would you want to build so many of a particular unit that you would set to repeat?
Um, always?
When I'm gearing up for war #1, my production city/cities are on Axeman repeat. Once I have plenty of axes and am ready to prepare for a period of peace, they go into Archer repeat. For war #2, it's usually repeat for a series of Axe/Mace, Cat, Cat, Archer/Longbow.
I even use it when I'm going into insane settler expansion. Repeat: Archer, Settler. Or when I'm spreading religion: Repeat missionary.
Saves so much time not having to choose a new build every couple turns.
Back to the original question, no way to simply change order. I really wish there was a drag and drop ordering, but Ctrl adds to the top, Shift adds to the bottom, and Alt (alone or in combo) is the repeat function. So if you want to put something in the middle, you have to delete whatever is below it, Shift-click, then replace what was below.
Also, remember that hammers are kept for partial builds for something like 10 turns, and begin to degrade after that. Very handy for when you're nearly done with a building but need a military unit quickly.