After a Contract is agreed and signed off, if further amendments are required, you can create a second Opportunity and link it to the originating Opportunity which will sum up the total amount to the Parent's Opportunities' linked Contract
1. Create an Opportunity with Product.
2. Fill in the 'Parent Opportunity' field to link it to another contract
3. Click the 'Parent Opportunity'
4. Click the Contract
5. On the Details, it will sum up all the total amounts for both contract