Volume Transitions -- Automation or Compression?

That is a mixing decision.  Depending on the context, you may want the solo instrument to get quieter.
 
Mastering is NOT supposed to change what your mix sounds like.

I use automated volume envelopes for musical transitions. They bring things in and out of the mix, quite abruptly in some cases, and also adjust the levels during certain parts.....
 
For example, in a chorus, I may want the B3 organ to come up under the mix and be more prominent in the mix....a volume envelope does that just fine.
 
Compression is what I apply generally to the entire mix, but mostly lightly.  No heavy compression for me and I NEVER use compression to control volume levels. Never.
 
My tracks all have volume envelopes in them. The pic below is the piano fills in the first verse of The Best Christmas....   here>>>  http://soundclick.com/share.cfm?id=11971116
 

 
I don't often have an instrument solo while everything else goes silent in my music.... but... I don't think I would necessarily do anything other than mute the others using the envelopes. That solo instrument, MIGHT need to be pulled UP a few db to compensate for the loss of the other instruments, BUT it would also depend greatly on what the instrument was.  What I mean there is that a more delicate instrument like a mandolin might need a few db gain, where drums & bass, for example, would probably be just fine with no changes to the volume envelopes. I seem to recall doing something like that in a song in the past..... but no compression gets used specifically for that, just envelopes. Envelopes are your friends.

Thanks a bunch, folks