Editing Constitution Class (section)

===Upgrades===

Only one turbolift serviced the bridge of the original configuration Constitution-class ship. In the late 2260s, some were refit with a second lift on the port forward section of the bridge. At the same time, an automatic bridge defense system was installed. (TAS: "Beyond the Farthest Star")

After the major refit in the early 2270s, the bridge aboard Constitution-class vessels would continue to utilize two turbolifts, but both would be located behind the command chair. (Star Trek: The Motion Picture)

The Star Fleet Technical Manual, states that the original bridge was rotated 36° port, which would have the turbolift shaft travel down the centerline of the saucer section. The Technical Manual gives no reason for this offset. With artificial gravity limiting any sense of momentum and electronic displays instead of windows, there may have been no reason for the bridge crew to face directly forward. 

According to production designer Matt Jeffries, dramatic necessity required the turbolift to be moved from its planned centerline position (as seen on the Enterprise model). This off-set placement allowed characters to walk onto the bridge and enter scenes without being blocked by actors sitting in the command chair. 

The bridge of the Constitution-class starships were subject to many minor and major cosmetic changes over their many years of service. In particular, the main bridge of the USS Enterprise seems to have undergone considerable changes in appearance over the years. Most notably, its systems were fully upgraded along with the refit of the early 2270s. (Star Trek: The Motion Picture)

The bridge underwent only a few minor modifications from that point until the destruction of the ship in 2285. The bridge of the USS Enterprise-A, commissioned one year later (in 2286), had mainly cosmetic differences at launch, but, by 2287, it had been drastically upgraded to reflect the advances made in computer control technology.

The bridge module had again been replaced by 2293. The lighter color scheme of the original Enterprise-A bridges had made room for a darker, more militaristic look. (Star Trek II: The Wrath of Khan; Star Trek IV: The Voyage Home; Star Trek V: The Final Frontier; Star Trek VI: The Undiscovered Country)

The controls in the fifth movie were designed to reflect a change towards a more TNG control style, but in the sixth movie the controls went back to a more push button design. While this has not been addressed in the canon one might theorize that the controls were replaced for any variety of reasons.