NCC-1778 U.S.S. Atlantis
U.S.S. Atlantis, NCC-1778 was one of the new-build Enterprise subclass vessels built in 2274, from Utopia Planitia Fleet Yards. This starship, along with others built and converted at the same time, was learning from the lessons learned from the shakedown of the refit Enterprise 1701.
Starfleet was undergoing a major change in policy and technology. This was the New Technology era of transition from the old circumferential warp starships that dated back to the 2240s to the new linear warp drive technology. Intermediate technological developments of the Achernar, Endeavour and Constitution (II) subclasses help to upgrade the starship technology to learn from the lessons of the 2260. Enterprise subclass technology was the aspired aim, but the refit of dozens of Bon Homme Richard and newer Achernar variants of the Constitution class was going to take time. As an interim measure, the less capable Constitution (II) and Endeavour variants would allow the fleet to be upgraded without taking too many ships out of service.
The Starfleet uniforms had been changed in the end of the 2260s for jumpsuit variants. By 2278 the uniforms would be changed again to the maroon uniforms that would be synonymous with Starfleet for the next eighty years. As the United Federation of Planets was expanding, so the need to explore and colonise became more and more important. Starfleet Command knew that they needed to upgrade the fleet quickly. Building a whole new class of starship, learning from the lessons of the Constitution class five-year missions of the 2260s, was the preferred option, but this would mean that dozens of earlier Starfleet ships wold be obsolete. Upgrading the fleet was needed urgently and rapidly, in light of the urgency Starfleet Command authorised the rebuilding of earlier Constitution class variants with the new emerging technology.
U.S.S. Atlantis was one of the first Starfleet ships to benefit from the full array of this new technology, immediately being assigned to the forefront of the Federation border, to push back the boundaries of unknown space and to showcase the new capabilities that Starfleet had, as a deterrent to both the Klingon Empire and the Romulans Star Empire as well.
With Star Trek: The Interim Years I want to have a coherent history, with a realistic backstory as well as the present. The Atlantis represents the beginning of the Enterprise subclass new-build construction and the Discovery is the end of the Enterprise and beginning of the Levant subclass. The Atlantis represents the 2270s period, of the introduction of New Technology and linear warp drive as well as those Motion Picture uniforms.
Starfleet policy was in flux at this time, modernising from the traditional to the new. The starship class all had individual emblems, but by this era that policy is being questioned, ultimately being abolished. The Wrath of Khan uniforms are introducted by 2278, as seen on the Bozeman in TNG: Cause and Effect. Equally the Motion Picture uniforms are only seen in the one film set either 2271 or 2273, depending on what year you set TMP in (2273 for me). Arguably the uniforms are introduced in 2271 as the 110th year of the United Federation of Planets, shortcomings mean the uniforms are replaced by 2278.
The justification of refitting almost one hundred starships is a necessity to explain the Enterprise refit, but in reality only happened on rare occasions - like the rebuilding of the 2 Royal Navy Lion class cruisers Tiger and Blake to add helicopters. It would be quicker to build a fleet of Constitution (refit) ships than strip, breakdown and rebuild over fifty starships - a task that would take decades and is the 'justification' for the Phase II Constitution II and Endeavour subclasses as quick-fix temporary solution until they can all be refit/rebuilt to the new design.