Hello bluemyndi
Is the storage cage shown on the strata plan, or does it just show a car space? Either way you could apply to your Managing Agent or Executive Committee for their sanctioning its removal in the way you propose.
Getting a swap with a visitor space requires a lot of hassle which I don't think is worthwhile, and it would be subject to a special resolution at a general meeting which may well be rejected by the other owners.
Without knowing exactly what the strata plan shows, it's impossible to answer whether the developers did something which is legal.