Change orders only occur for estimates that have already been approved. You can make as many changes as you wish to an estimate that has not been approved by your customer.
In order to approve an estimate, simply select the Approve icon as shown below.
Upon approving an estimate, the icon will change to allow you to make a change as shown below.
You can make a change by selecting the Change Order icon which will then create a brand new estimate tied to the job as shown below.
Notice the above Status is ChangeOrder.
Saving this document will create its own separate document tied to the job that can be separately approved and invoiced.