Not all has to be "paid" up-front, but once any amount for upgrade is "put down" item is de-activated fully (excluding in clan war) and once full upgrade cost is paid in full, upgrade then begins to take place.
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Not all has to be "paid" up-front, but once any amount for upgrade is "put down" item is de-activated fully (excluding in clan war) and once full upgrade cost is paid in full, upgrade then begins to take place.