Why do you think it's unprecedented? This is a thing that happens all the time. Younger players commonly break into the league in relief, and it's also not exactly rare for older players with the right skillset to move to the rotation. Lots of arms also obviously bounce between roles at first.
I'll let you define good vs. great, who qualifies as a "reliever", and set the definition of "around his age" but:
Tanner Roark
Carlos Martinez
CJ Wilson
Marco Estrada (after 2011)
Drew Pomeranz
Chad Bettis
Danny Duffy
Jason Hammel (2008)
Carlos Carrasco
Junior Guerra
Lance Lynn (every other good Cardinal SP breaks in as a RP)
Justin Masterson
I'm sure there are many, many more notable examples if you go back more than a few years.\
Personally I think Tanner Roark and Biagini compare very well in terms of talent, stuff, ceiling.