Without the requested information, I can only provide you with the following general response, which will hopefully still answer your query.
How you go about this will depend on how long each affected employee has worked there for. These with less than 2 years’ service are the easy ones – they have no unfair dismissal protection which means you can simply issue them with notice as per contract to terminate their contract and then re-issue the with a new one incorporating the changes. You just have to ensure you honour the current terms during the notice period.
Those with more than 2 years’ service will require more attention. You should consult with them to start with, explain the proposed changes and reasons for them and ask them to consent to the new terms being introduced. If they do, great – you then introduce the terms as agreed.
If they decline to accept them, your option is to leave them as they are, or still issue them with notice of termination and re-engage them on the new terms. The risk there is they may claim for unfair dismissal and you would have to provide a justifiable reason for dismissing them in the first place, such as how important these changes were to the business, could you have continued without them and how imperative they actually were, etc.
Hopefully the majority of employees will fall in the first two camps and you can change them without any real risks.