There’s a similar and newer pull request FEATURE: Array re-ordering in Editor if Tynril’s doesn’t work. It looks like neither will get merged because Epic wants to use drag handles instead of up/down buttons.
Just tried this in 4.18 and it’s not always working. If the array is set to Instance Editable and you reorder it, it will break the array, add new empty items to it. It will show as if the array is fine in the editor, but internally, when the game is running, the array will be different. This will leave the array in a permanently broken state, I found no way to fix it other than to just a git reset --hard HEAD to the previous commit before I tried reordering the array.