Replacement of purine with another purine base is known as

The replacement of purine with another purine base is known as Transition.
Transition is the substitution of a purine from another purine base or pyrimidine from another pyrimidine (C⟷T or A⟷G).
A transition is a DNA point mutation that substitutes one purine for another or one pyrimidine for another, all other point mutations are called transversion.