When switching mattresses, a break-in period where your body adjusts to the new support is common because the materials have not had time to soften and conform to your body.
It is recommended to try a memory foam bed for at least 30 days, and why the 365-night trial period is valuable.
The old mattress you've been sleeping on for years has adjusted to the shape and size of your body as well as your sleeping positions. After years of repeated use, materials compress and no longer adequately support your body. That indentation left by your body may cause sagging, reducing sleep quality.
A new pair of shoes often requires a breaking in period. The same principle applies to your new mattress; it may not feel comfortable until the materials have stretched and softened.
The break in period allows time for your body to adapt to your new mattress, but also for it to adjust to you. If your old sleeping surface was beginning to break down, your body might have over compensated to make you feel comfortable. A new sleeping surface means your body may have to readjust to the support of your new mattress.