With the Aldi SIM if you buy a new SIM which can be pushed out for Standard, Micro or Nano SIM, you then go into your Aldi account and set the new SIM number (not phone number) for your phone. This deactivates the old SIM and the new SIM works as your old number, the added benefit is the $5 you paid for the new SIM gets added to your balance. DO NOT activate the SIM first before this or a new number will be allocated to the new SIM.
If you do the change-over by phone you may lose the $5 but I do the change-over online.
To do this:
login to your account
click the "Manage Account"
from the list click "SIM Replacement"
If you have multiple phone numbers a box will drop down asking you to chose the phone number you want to change the SIM for. Chose the correct number.
if you have chosen or don't have more than 1 number put the new SIM number from the card into the provided box.
click "Replace SIM"
You have successfully replaced the SIM