Why older cruisers remained useful after the first battlecruiser

Did the first battlecruiser immediately make every armored cruiser obsolete?

It made the concept obsolete, but older ships remained useful until battlecruisers existed in sufficient numbers and were employed properly.

A single battlecruiser could make the armored-cruiser concept obsolete without instantly making every armored cruiser useless. The same distinction applies to HMS Dreadnought and existing pre-dreadnought battleships. Older ships retained comparative value until enough new construction appeared to displace them. Doctrine and employment still determined whether a theoretically superior type delivered its promised advantage.