All companies, and specially consulting and IT companies, change people from one position to other by different reasons.
For more defined roles or less complex roles, the transition from one person to other is smooth and there is not major issues for the new person and for the quality of the outputs delivered from this role.
But for some other roles, where the complexity is high or the relationship component is key, you cannot change a person in 15 days.
For sure there are things, as an employee leaving the company, that cannot be forecast, but for other situations, there should be a potential succession plan for key roles and key people that enable the company to maintain the quality levels and keep the relationship with the customers.
I have seen succession plans initiatives in companies, that first their attrition is very low (for sure not an IT company), where they evaluate people to detect high potential employees, then define a career path forward, and then a set of activities are done across the year/s. Finally the evolution is measured.
Easy thing?
Not at all, this implies the company has a clear direction and strategy in terms of human resources. This also implies the company takes seriously the human resources processes, being senior management the people leading the execution of these processes.
There are so many companies making advertisement about how important are for them their people, but there is a small amount of companies taking care of it.