Portrait Artist Forum - Is it ok to make changes once varnished?

:! I just recieved an email from a client that said her husband wanted more color in the background. She saw the painting several times over the internet and was completely happy with it. This painting was done last summer and I varnished it before it was sent to her over Mother's day weekend. I want to tell her no! I'm so tired of that thing...but, she hasn't paid me yet. So, is it ok to make changes to a painting once varnish has been applied and if so is there anything special that needs to be done?