Matt Serra. 
I feel like mighty mouse should be mentioned 
Josh Barnett developed some good power later
Frank Mir too, he was basically a submission guy exclusively then he range Nog's bell and dropped Kongo like a bad habit
Lauzon developed some good pop after years of not being very heavy handed
Tyrone Woodley, Machida.
Demian Maia. Big improvement with his hands. The way he was lighting up Munoz until he gassed was impressive as hell.
Fedor developed KO power with stricter testing. GOAT.
Brian McLaughlin -Really? He KO'd jens pulver a long time ago.
Lauzon developed some good pop after years of not being very heavy handed
Roy Nelson, Frank Mir, and Dan Henderson are great examples. It's crazy to think that Roy Nelson hits as hard as any HW, and he's only been training boxing and trying to land powershots for a few years.
Hendo and Mir were always powerful men, but they didn't have any sort of refined punching early in their careers, they gravitated towards other things first. Once Hendo, like Nelson got his technique down, he became one of the best bombers in the sport.
Thanks for the old school UG MMA convo. Fuck the TMZ bullshit. Let's talk MMA
Machida. The Thiago Silva fight being his turning point
Also how about Werdum?
Werdum is the best answer
DHK (Dong Hyun Kim) comes to mind. He got reckless like many do after discovering KO power.
Machida is one of the best examples.
Like truevalon said, something changed in the Thiago fight. Before the Thiago fight he had TKO stoppages but they were fue to exaustion and cuts (doctor stoppages) but no one punch KO's and often against much smaller fighters ( BJ had the balls to fght a 225lbs Machida).
I like the Machida example too. I think it's just a matter of him committing to his shots. It looked like he used to pull back on his punches and just concern himself with placement of the shot, then back to defending, when he sat down on those shots, heads rolled.