Hi Mermla,
Some of the suggestions in post #2 here may be helpful.
I would give it a month or so to give the mattress time to break in before making any exchanges or adding a topper.
If after a month or so you do decide that you need a little extra softness then exchanging for the Contour Signature may be a little less “risky”. Both the Signature and the Select have 4" of 5.3 lb memory foam in the comfort layers but the top 1.6" in the Signature is the softer formulation so not only would it be softer but you would be a little closer to the polyfoam support layers (less memory foam to sink in to) than you would if you added an inch of memory foam to the Select which may be a little less risky for alignment than having an extra inch of memory foam on top of the Select (which would be a total of 5" of memory foam). In addition to this you can test the Signature in person to make sure it works well for you and that you like how it “feels”. There are many different types of memory foam that each have a different feel and response (even if they are the same density/quality) so you wouldn’t know how your mattress feels with a specific memory foam topper until you’ve had the chance to sleep on it. In other words … adding a topper is a little bit more of an unknown.
Of course the downside to upgrading to the Signature is that it’s more costly than adding a topper.
So which one is best for you would depend on whether the lower cost of adding a topper was worth the added risks to you.
Phoenix