If your sales order has subsequent documents then it is not possible to link to your old contracts. For such orders this option will be disabled.
For those order that don't have subsequent orders:
Goto VA02 T.Code.
Enter the order number.
Enter.
Ensure that the customer numbers are same in both. If not change the customer number in order.
After that Click on Sales document --> Create with reference.
While changing the customer the taxes will be re-determined.
Enter your contract number.
Click on copy.
New material will be added here to your old one.
If you want then you can keep this or delete the material of your order
and then save.
Or
You can assign your sales order to contract also.
Goto VA02, Enter your order number.
Edit --> Assign contract.
Here you can assign either Header or item also.
