First I installed 90 day demo version in my laptop. I developed the project in that demo version. I purchased the Developemnt License key.

So, I deactivated the demo version. Now I am trying to activate it again using the licensed key(25 Chaaracter). But it is only giving option to enter 16 digit key.

Suggest how to activate it + I also want to use my existing changeset file developed on 90 day demo version.

You will receive a key with the trial version email. Have you tied that?

Your changeset from the 90 Day trial should work with your Development license.