TL;DROne sentence to describe your idea with a bit different words than in the subject
when you click on a request slot when there is already a request slot move the request location instead of giving an error
What ?when in the invintory looking at the logistic menu, click on an empty box and request something already requested in another slot. an error will open with 'this item is in another slot'. my suggestion is to instead open the item request menu with the current configuration (lower/upper limits set as they currently are) and if the user presses enter/ clicks the green check to apply the change then put the item request in the new slot and empty the old slot.
Why ?I think it will increases the value of Factorio as a game because it will
a) allow for much easier rearrangement of the request slots,
b) remove the need to tediously search dozens or (in modded games) hundreds of items to find the item whose request you want to remove.