I have read several books on solar evolution but I'm interested in one particular event. When the sun switched from gravitational/deuterium fusion to thermonuclear fusion, how did the energy output increase affect the sun? Did it expand or did it shot out large amount of mass off into space or was the changeover so slow that it did not affect it at all? I would appreciate an answer to this specific question.
Are you aware of any publication that deals with this particular event?
I think your question can be answered by a Hayashi Track diagram - see http://coolwiki.ipac.caltech.edu/index.php/Pre-main_sequence_evolution
. Unfortunately, on-line descriptions are not very complete - I suggest you consult a good text such as "Modern Stellar Astrophysics" by Ostlie and Carroll. In section 12.3, they give a good description of pre-main sequence evolution.
For a one solar mass star, deuterium burning starts at about point 1 (Hayashi track). This ignites first (over p-p fusion) since it has a high cross-section at low temperatures. But since there's not a lot of deuterium, this stage of ignition has little effect on the star - it just slows the rate of collapse slightly. By point 3, the star is still shrinking, and the other proton-proton chain nuclear reactions start up, along with the the CNO chain. The core doesn't start expanding until point 5, but the stellar atmosphere continues to collapse. The gravitation energy is slowly dwarfed by the nuclear energy, and the star settles into main sequence. There is no great release of mass to space when nuclear ignition occurs - the collapse merely stops.
Hope that helps.
Prof. James Gort