I did two experiments around the same time a week ago on two different unrelated and unconnected aged websites. Previous titles had been set once years ago and never changed, except, in one case, a single word, which had changed annually without apparent loss in ranking.
Changed the home page title tag to a two-word phrase which matched the domain name (down from about eight words).
Result: Ranking for that phrase dropped from p. 2 to p. 8.
Changed the home page title tag to a single word which matched the domain name (down from about eight words).
Result: Ranking for that phrase dropped from p. 1 to p. 8.
* Short-term losses are to be expected from changing titles.
* Google may see it as an attempt at optimization.
* Google may see too-short titles as a bad signal.
* Minor changes don't necessarily cause problems.
* Major changes could at least temporarily lower trust.