According to Rule 10 of the Magistrate Court Rules a summons lapse after one year. In terms of the High Court it is different and the summons never lapse and you will have to withdraw the summons.
In your case, the previous summons did indeed lapse according to the rules. The re-issue of this summons, and service thereof should have been done at your new address. The Magistrate Court Act does however make provision that the summons could have been served on the previous address if the reasonably could have believed that you still stay there. They could have then merely attached the summons to the property.
The problem now is that judgement has been granted. In order to apply for a rescission of this judgement it will mean that you will have to show two things:
1. You did not receive the summons and was not aware of the action taken against you, and in this case I think that on this point you will be successful;
2. That on the merits of the case, you do not owe the amount claimed and that you have merits to defend the matter. Here I foresee a problem as you categorically state that you do owe the money. If you admit this, the court will not rescind the judgement.
My view is that you should try and convince the plaintiff and his attorneys to allow you to make arrangements again to pay this amount off. If they refuse however, they will be able to proceed with a warrant of execution against movable and immovable property.