A very important thing to observe in this regard is the perfect propriety (adab) towards the ex-Shaykh. One should continue the same extreme humility and politeness, with no backbiting or hard feelings (kinah) at all. These are the rights due for each and every Muslim.
This holds even if he is a false (ghair kamil) or innovator (bida’ti).
It was his company that kept you practicing the essentials of the religion in that period and eventually guided you to the truth.
You owe him some respect for this, at the least!
Related: