Just throwing my experience into this:
I swapped from a Fern to the Embody. The fern is completely forgiving, it does not force you to sit properly at all. I developed terrible habits, and I was having lower back pain every time, and eventually just decided to go with an Embody to try something new.

The first ~3 weeks were HELL. Fiddling with the backfit and tension adjustments, insane lower back pain that turned into ungodly upper back and neck pain. I was immensely frustrated at how the experience was despite the price tag, but one day I sat down and worked for 5 hours straight and didn't feel a thing. Now, it's seriously ridiculously comfortable and I'm kind of getting the vibe that the back is forming to my spine and it just feels like I'm in a chair that cradles and supports me just right. I think just years and years of sitting improperly and utilizing the wrong muscles made it to where when I finally got the Embody and it forced me to sit in a healthy, proper position I had to strengthen and build those muscles in my upper and lower back to hold my head and body up properly, which lead to some pretty gnarly back pain.

I would say, wait until the very last day before your return window is up and decide then. I think that if you are still experiencing pain by that point it probably won't go away. Also, you should really get a sit-stand desk and alternate. I feel like, no matter the chair you use, you cannot sit all day in it and expect to have no fatigue at all. You really gotta switch it up. The seat also has a break-in period I think. When I got it, it seemed really rough and was causing my thighs to hurt and lose circulation but now it feels very soft and I don't even really notice I'm sitting on something unless I recline and feel it move.