When in the course of construction you need to get a price for a change in the Work, here's how. If the project is private, you have choices depending on how formal you feel you need to be. Generally, a formal paper trail is good to have. But if you have a good relationship with the Owner and Contractor, and the change is simple, you might even make a verbal request. An example is changing lay-in ceiling panels from one model to another. Very simple; verbal works. Or perhaps an email would suffice. The email will document the change for future reference. A long email or a letter describing the change with a clarifying sketch can take care of a more complicated change.
But if your project is public work, being informal might come back to bite you. So formal is better. I believe the AIA has a form you can use. We dreamt up this form below for a small public project that was, uncharacteristically for us, becoming adversarial.
Note that it needs to be clear that you just want a price, and only afterwards will a Change Order be issued (if the price is acceptable). In many instances, you might be chasing someone's dream that doesn't have a direct role in the project - the boss's boss - making the situation a bit political.
The formal approach always works, and it takes just a few minutes longer to implement.