Can someone tell me the different between a TCA and TeCA antridepressant in regards to symptom relief for Fibromyalgia?

I have been on Nortriptyline for 10 years and recently started getting bad side effects from it. I asked me doctor if I could try switching to Mirtazapine because I heard this drug does not disrupt REM sleep in the same way and does not have some of the side effects I get from Nortriptyline such as urinary retention. I took 150mg/day for my Fibromyalgia. Will I get the same benefit out of this other drug even though its a TeCA? The TeCA class seems virtually unheard of online. I'm just wondering if I would be able to discontinue the Nortriptyline altogether.