For contracts longer than 12 months, you should have a clause to allow unknown technologies to be added. Particularly for high tech and IT projects.
Part way through a long contract, the industry best might have superseded some of the items specified in your contract. You need a way to change to these newer technologies. They may perform better, and at a lower cost.
If you don’t have a way to add these, you could be stuck trying to source a specified item that you know is outdated, cannot be supplied, and will be more expensive to maintain because no one supports it anymore. An addition clause will allow you to approach the client with a change/addition proposal that the client can comfortably approve. If no clause exists, they may not be able to allow a change or addition because of their internal organizational procedures.