Break the news of promotion directly to your friends
When promoted, you should be the first person to convey the news to your friends. This might feel awkward at first, but go ahead and do that because if your friends find it out from another source, there is a good chance they will think that you have changed already. The best thing would be if you can invite them to a dinner and celebrate the occasion. This will also be a good place to address them about the expectations both sides should have with each other.
Be Objective and Fair in your dealings
Be the person with power does not mean that you start doing your friends special favours. This would automatically result in suppression of rights of other employees and the time wouldn’t be long when everybody starts to despise you as a biased boss.
Ensure that you inform your friend about his shortcomings
Although it may sound difficult, but do not shy away from addressing the shortcoming of your friend. Of course, it should never be done in public, but the important thing is that it has to be done. By doing this you will not only be doing your friend a favour, but will also be honest to your service.
Do not commit the mistake of doing favours to your friends while evaluating. They should know beforehand where they stand, and the final result should not come as a surprise to them. Having said that, do not back off from giving them their due, even if other people raise doubts.