at the beginning of the game when you bind your support as kalista and then your support dc's when you buy the Black Spear from the store you cannot bind another person. when you try to it makes a loud gong sound and goes on cooldown and you are not soulbound to your new target.
I believe it was intended that way, once binded to someone you cant change the oathsworn. Otherwise it would be abused i.e. you have a sona + kalista bot lane and nasus top, then you could bind with sona to have the strenght of passive in lane and then after mid game hits you could tell sona to dc/reconnect and bind nasus to have him as an initiatior and to give him a free gap closer
