Schedule of work - Update project estimates

Hello,

 I have the following query about schedule of work.

Scenario:

I did a schedule of work on a project.I added my work items on a new sub project -sub project 2.

However after the validation/calculation/ and approval proceed, when i try to update the project estimate i am having the following message:

Note that i do not want to update my existing project estimates made on sub project 1.

Is it normal that i can have only ONE schedule of work per project ?

Thanking you for your feedback.

BR

Lakshmi