Understanding how to modify a public procurement contract by means of a rider

The article lists the cases provided for; a contract may be modified without a new competitive bidding procedure when:

"1° The changes were provided for in the original contract documents [for example: price variation clause] ;

2° Additional work, supplies or services have become necessary ;

3° The modifications are made necessary by unforeseen circumstances;

4° A new contractor replaces the original contractor;

5° The modifications are not substantial;

6° The modifications are small.