When you have an exemplary employee. Someone who you rely on for their judgement, brains, talent, it's quite normal to promote them. Give them new challenges, watch them grow even more.
The ability to not think of yourself, but think of the betterment of the organization by promoting them is what all managers should do. To not pigeon-hole your employees and leave them stuck in a position they have outgrown, shows planning and maturity.
But there comes a point when you need to let go.
Not everyone in the working world works the same. There are no two people, no matter if they have the same education, skills, experience, etc. that will produce two identical people. You may have two people with similar skill-sets, similar desires to succeed, similar ways of thinking, but they will never be IDENTICAL.
When you make the decision to promote an employee, you have to be willing to let go. You have to recognize that the next person who comes in to fill that position will not be the carbon copy of the person you just promoted. This new person may have more experience, less education, a different history, and all individuals are different. Period.
You've faced a tedious process in interviewing potential employees, maybe even test-driving one or two to learn it wouldn't work out, and you now have a new employee.
There will be a learning curve as you get to know each other. The new person may make mistakes. The new person may go to their predecessor and ask for advice. But they will never BE their predecessor.
You have to have faith that you hired that new person for a reason. You know they're qualified, you have a feeling on their work ethic, their commitment to the organization, and you have to believe that the new employee is trying the best they can. They are trying to assimilate to a new company culture, trying to feel YOU out as well, trying to learn all the intricacies of the job their predecessor had down pat.
But you need to let go. It's unfair to the new employee, the predecessor, and yourself to expect that this person can come into the job and be that other person. You have to respect that everyone has different ways of doing things, and that doesn't mean it's BAD, it's DIFFERENT.
You need to let go. If you won't give that new employee a chance to show you what they can do, then in all fairness to everyone, you shouldn't have promoted the predecessor.