Why dichromate changes into chromate in basic medium?

The equilibrium will shift to left in excess of H+ ions. Hence the more acidic the solution, the more the equilibrium is shifted to the left towards the dichromate ion and the colour of the solution  is orange.

When we add OH-  ion in solution (increasing the pH), it will react with H+ ion present in solution increasing the concentration of H2O.  

As the concentration of H2O will increase, the equilibrium will shift towards right and the concentration of chromate ions will increase and the colour of the solution will change to yellow.

 

Cr​​​2​O7_2-  +OH-   -----> Cr2O42- +H2O
This conversion can be explained by the fact that chromate and dichromate ion exits at equilibrium at a pH of 4.
Cr207_2- +H2O Cr2O4_2- + H+