Herniated disk -> back pain-sudden change in pain?

There can be changes. Disc herniations can present in various ways & change their pain pattern as they heal or progress or even recur as in 5% of cases. They can initially present as back pain & then radiate to a leg(s) & back pain will improve or stay the same. Leg pain can start as initial pain & improve & back pain can get worse & then resolve (centralization). Sometimes there is no pain, just neurologic issues.
HNP effects vary. Herniated disks (hnp) can be silent (no symptoms). Others cause back pain (probably when there is a fresh tear of tissue). Others result in nerve problems: numbness, tingling, muscle weakness, and /or pain in the leg. They can even interfere with ability to empty the bladder and bowels (an emergency).