Change a mob into another after a set amount of time

Create a procedure for when the entity spawns, and set an nbt number value for your mob, set it to however many seconds you want the timer to be times 20 (20 ticks a second in Minecraft).

Then create a procedure on entity tick for your mob, and decrease that same nbt value by 1, and in the same procedure use an if statement to check if the timer is below 0, and despawn your mob and summon the new one