Mix/Transition audio at any given moment?

Hi everyone!

I'm going to ask this the best I can as this seems like a pretty tough question/topic. Would it be possible (through Unity) to mix and transition two audio samples flawlessly? For example, your intro/main music is playing a classical piano song, then when the main player character hits a power up the song suddenly has a more vibrant/modern sound to it without there being any gaps or difference in musical notes being played. It would just transition into the new song for a set period of time (while the powerup is active/lasts). Is this even possible in Unity? Or, would you be looking at having to build your own engine for a listener of sorts that would mix/insert the audio at the right moment?

As I said, I know this maybe a tough one. If it's kind of a crazy idea - I completely understand. The original/classical idea would be just to transition into a completely different music sample once a "powerup" is activated.