Under normal circumstances, as long as funding is available and there is a new model available, then patients are upgraded every 5 years. If a processor has been lost during this time then the upgrade would then be missed as the funding would have to be used for the replacement (See lost processor policy for more details).
When the upgrade is due, a letter will be sent out to ask for choice of colour and could also have a choice in type of processor and accessories. It will be a good idea for you to visit the manufacturers website in order to gather information before making your decision on these, so that you make the right choices to suit your needs.