You don’t need a DA unless you are planning to radically change the use of the parking space to something else that wasn’t approved. To make this lasting and permanent, I would think a 99-year mutually beneficial lease signed by both parties and attached to your title deeds by your conveyancers might do the trick.
Obviously, the only problems likely to arise would be when one or othe unit changes hands.
If the parking spaces are common property, the owners corp could simply agree to the reallocation. I don’t think you’d even need a bylaw (but I may be wrong).