If I sit up in bed, will I slide down too much in an adjustable bed?

It is natural to slide down as far as the bend of the bed when the head of bed is raised. To prevent excessive sliding down, be sure to raise the lower section of the bed at the same time, or before raising the head of bed. This adjustment will keep the hips from sliding further.

If I sit up in bed (non-adjustable), will I slide down too much?

It depends. If you slide too much, place a towel under your hips to increase friction and stability. Remove the towel before sleeping to ensure easy movement is restored.  Alternately, place a pillow under your knees – or a small roll under the sheet at the level of your buttocks. This will block you from sliding excessively.