The prototype vessel, the USS Excelsior (NX-2000) was designed to test Starfleet's new transwarp drive. The drive would prove to be a failure and the class was equipped with standard warp drive and the class was originally classified as an explorer.
While the class received a refit in 2293 the original configuration proved to be highly adaptable allowing it to remain in service well into the 2370s. Though by then the design was reclassified as an exploratory cruiser.
The refitted Excelsior had several design changes over the original such as the inclusion of a a second impulse engine at the rear of the saucer section and redesigned warp nacelles to improve the ship's performance at warp speeds.)
However, these design improvements did not prove to be worth the extra costs in constructing the newer version over the original design and the vast majority of Excelsiors constructed were in the original configuration.