Visually, missile turrets only underwent a few updates in regards to their StarCraft predecessors. After a few art revisions the missile turret was done finally and wasn't revisited again until the end of development. However, when the armory in Wings of Liberty was finalizing the upgrade changes of the artwork, the missile turret was picked for a visual upgrade. The titanium housing upgrade simply added more chunky armor bits to the existing turret, but with a newer texture pass.
During the Legacy of the Void beta, missile turrets lost the engineering bay requirement for their construction. This was reinstated, as the developers felt that the change didn't spark a change in strategy, and that "the choice to just build Turrets easily didn’t feel like a good add to the game."
↑Yes, I can confirm that some units get visual upgrades when you boost them through research.
Zerglings, Marines and Banelings are just some examples though, as there are other units that are getting a visual upgrade.
For example, Battlecruisers have the ability to choose between three specializations:
- Defensive Matrix (a shield that absorbs damage)
- Missile Pods (Air to Air AoE damage)
- Yamato Cannon (honestly, do I need to introduce this one? :) )
Each one of these specializations changes the graphic of the battlecruisers, thus making it easily recognizable.
Just to clarify things up: you can only have one specialization per Battlecruiser, and once you choose you cannot switch to another one.
But not only units are affected by this visual upgrades, even buildings are being visually upgraded.
Let's talk about Bunkers: when the NeoSteel Frame upgrade is completed (+2 cargo space) the bunker actually gets bigger and taller. Zhydaris. 2009-12-01. Visual Upgrades?Battle.net StarCraft II General Discussion Forum. Accessed 2009-12-01.