mnieman

the flight to quality logic is correct, although your interpretation is wrong.

for a start, aaa bond yields will be lower than b rated bonds.

when flight to quality happens, investors buy more of the less risky (aaa) bond, price rises, aaa yield falls. in other words, you will buy the lower yielding (aaa) bond and not sell it as you mentioned.

similarly, investors sell the riskier (b) bond, price falls, yield rises

hence the spread widens