In Project Server (2013 if I need to be more specific), assume you have the following Enterprise Resources:
- Res. A
- Res. B
And you would like to replace them to be a single Enterprise Resource:
- Res. C
What's the best approach to do this? And how would you replace all task assignments of A and B by C?
If you rename A in C that's taken care of for res. A.
You then only have the assignments of Res. B that still need to be reassigned to Res. C. In MS Project you could then try to replace Res. B by Res. C in each project that has assignments for Res. B.
But since Res. A is already renamed to Res. C, Res. C is already added to the Project Resources pool and hence cannot be selected as a replacement for Res. B.