Armored Core's gameplay did significantly evolve over various iterations anyway, mostly in terms of how fast, free, and open the gameplay was, so the earlier PS1 installments were pretty slow and mostly ground bound while Armored Core: For Answer is insanely fast and almost entirely airborne.
Each numbered Armored Core from Armored Core 3 onwards is set in a new continuity, and the same will apply to Armored Core VI. (Subtitled spinoffs are in the same continuity as preceding numbered entries, and early ones even allowed you to carry your save across between games.) Common elements between them are post-apocalyptic settings dominated by megacorporations which engage in conflict using highly skilled mercenary pilots such as yourself as proxies.