Have a look through some threads from Shibui recently (use the search function).
He discusses how to look after Japanese Black Pine (JBP) for growth or for ramification.
You ultimately need to consider which of these (growth / ramification) is what you so desire.
In terms of repotting most would do this during spring however I understand Autumn to be acceptable too.
All the best.
One of the fabulous things about growing bonsai is as you get old and decrepit your trees get old and beautiful