How to avoid engorgement...

Jess • Married to the love of my life, totally smitten with our little girl and boy.
...when switching from both breasts to just one per feeding. 
I saw my lactation consultant today because I have thrush, lovely, and while I was there she recommended going from giving both breasts per feeding to just one. Our little one has been having some gassiness and she thinks it's likely from overfeeding as I have a LOT of milk, haha. 
But, I just woke up with a very hard breast because, since I didn't use it in the last feeding, it hadn't been used for almost 6 hours. I used a warm compress and hand expressed some before she fed so that the breast wasn't too hard for her. But I don't want to end up with mastitis while my body adjusts to her new feeding habits. 
Should I hand express some from the unused side at each feeding just to relieve a bit? That won't signal my body to make even more, right? That's what I want to avoid for sure as I already have a lot of milk. 
Thanks!