Extending an existing contract

[#1]

I have an open contract with a 3 page website design. Based on my work, the client requested to do 2 more pages also.

My question.

1. Can I extend the open contract to add 2 more pages? If yes, how?

2. The price was $30 for 3 pages + 2 more pages = $20, the total will be $50, this will get justified?

3. As a best practice, what is suggested either to start a new contract for additional work or extend existing?

Thanks.

[#2]

The client can just add and activate another milestone to the current contract for the two pages at $20, no need to start a new contract.