Hi equestride,
I normally wouldn’t make any changes after only a single night because there is no way to know whether your experience is a result of an anomaly, needing to adjust to a new mattress, or whether the mattress needs a different combination of layering (the more different a mattress is from what you are used to sleeping on the longer it can take some people to adjust to it depending on “muscle memory” and the sensitivity of your body to changes in what you sleep on). Normally 30 days or at least a couple of weeks is best but if that’s not possible I would try and give it as long as you can so you can see if things are improving, staying the same, or getting worse over the course of the first few days. There is more about the normal break in and adjustment period in post #2 here. There is also more about the most common symptoms that people may experience on a mattress in post #2 here.
If it’s not possible to sleep on the mattress for at least a couple of weeks to get a better sense of how your mattress will feel over the longer term then the first thing I would suggest is a phone call to SleepEZ who will be able to help “talk you through” the types of changes that may work better for you.
I don’t know whether your mattress is too soft or too firm for you (too soft can lead to lower back issues if your hips/pelvis are sinking in too far and for side sleepers too firm can lead to “twisting away” from pressure which can also lead to back issues) but either way I generally wouldn’t put the firm layer on top … at least as the first change to make. Top layer is more about comfort and pressure relief and the deeper layers are generally more about support/alignment (which lower back pain often points to). If your mattress is too soft for you the most common change to make (based on very limited information and again I would have a more detailed conversation on the phone with SleepEZ before making any changes) would be to move the X-Firm layer higher up in the mattress so it was either the second layer from the bottom or directly under the medium layer which will provide firmer support.
Phoenix