As has already been explained, there is no config which updates the contract with change in vendor master, nor supersedes a vendor master data over contract to default payment terms in PO.
You can try this alternative, it will be slightly lengthy but easier than changing in each PO. Find out a list of all such contracts whose vendors have their payment terms changes through EKKO.
Then in transaction MASS, using object type BUS2014, change the contract's payment terms for each vendor one by one.
