Hi redbird,
The only way to know whether any part of your body is sinking in too far or not enough is based on any “symptoms” you experience when you sleep on a mattress. I would tend to wait until your experience has stabilized before making any changes to your mattress and if your lower back suddenly starts to hurt for a couple of days and this wasn’t happening previously and it has started to go away then it would most likely be from “other reasons” than your mattress.
Most latex mattresses don’t have (or need) edge support because latex is such a supportive material (see post #3 here) but for those that sleep with more of their weight on the very outside edge of a mattress or that are in higher weight ranges then firmer support layers or configurations under the softer top layer can reduce the feeling of “roll of” on the side of the mattress (and/or the feeling of “roll together” in the middle).
For most people though it’s more a matter of getting used to the “feel” of a different type of mattress.
Phoenix