It depends on how a ruler and a man he was before.After all, it is likely that he was stupid, arrogant, unfair to subordinates( and you as well).In this case - the rigor in full compliance with their obligations and, but no vengeance and malice.And business-politely explain: say, understand everything, there are different situations in life, but "nothing personal, it's just a business".
If you were in a normal relationship, then there is you can just friendly, but discuss the situation and agree on the same without familiarity - conscientiously perform their duties.
Do not know the reasons for lowering your boss, maybe not really match the previous post, and probably fell into a temporary disgrace and can again become your boss.
So, in any case - a prudent distance can not hurt.
I would have made him an ally and with his right hand with which to consult, as it was in this place and knows the ins and outs, and all sorts of nyuansy.Esli you earlier is not a relationship and the former chief of you as the saying goes, Ridicule, thenthen the relationship must be formal and polite, at least from your side.
You must be above all the squabbling and memories, because a person must be generous and able to work should be proschat.Na working relationship, and if the roles were reversed, you should be the boss with all the consequences and without panibratstva.Horoshie friendlyhuman relationships has not been canceled.
Respect as before.After all, he has experience.Make it your ally.Only without familiarity.Imagine how it is now all that unpleasant.It is better to be with him on good terms.After all, everything changes.