I just assume the Borg were always supposed to look First Contact style. After all, Locutus is done in that style in the flashback in the beginning of First Contact despite looking like the other drones in TBOBW. Also, the flashback scenes in Voyager's Dark Frontier also depict First Contact style drones.
Austin 3:16 wrote:
If the Borg were always supposed to look like how they did in First Contact in all of the TNG episodes they were in and were constrained by a lower budget of a TV show, then why did they decide to have a storyline reason in Enterprise for why the Klingons changed their appearance?
Because the change in appearance for the Borg wasn't as radical as the Klingons. When you get down to it, we still had the same basic Borg, just done in a different style. When the Klingons grew ridges, everything changed. Their uniforms, their culture, their behaviour, hell they even developed their own language. Something had to be done to acknowledge this.