The same two letters, in the same, consecutive, order can change each letter string into a word or each word into a new one. The letters will be different for each set.

 

1. SUP

    SAT

  

2. NVE

    MDN

 

3. PON

    VIL

 

4. COUR

    SURY

 

Answers:

 

1. YR - SYRUP

             SATYR

2. AI - NAIVE

            MAIDEN

3. YL - PYLON

             VINYL

4. GA - COUGAR

              SUGARY