i might be wrong, but the best time to repot all/most bonsai trees is in early spring, as manofthetrees said the root systems aren't to be disturbed to much and early spring is when all/most trees aren't as effected by repotting.
As the crassula is a tropical plant it can be re potted at anytime of the year. Normal bonsai, hardy species, are re potted in the spring, If the re potting is done correctly then the tree ia not effected by it,
Crassula can indeed be repotted any time of year they make roots very fast.
A lot of tropicals can but make sure they are healthy and growing. they also have sort of a dormant time were they don't grow that fast.
Hardy outdoor species can be repotted in spring or even in fall, but then they must be protected against extreme weather and to much water as they won't absorb much when dormant and the roots can easily rot.
Spring is the safest when new growth starts to appear.
