In various implementations organizations go for PPDS as solution. As a result resource master data selected for this is single or multi-activity (considering situation at that time). This means resources available in production environment (APO) are single/multi activity resources which are used by PPDS but not by SNP.
This comes from setting in ECC in transaction CFC9 where we define what kind of resource type to be CIFed to APO. Example Single Resource, Multi-resource, Single Mixed Resource, Multi-mixed Resource, etc.
Over a period of time if organization wants to implement SNP for their mid-term planning they need single-mixed or multi-mixed resources which could be used in both SNP & PPDS. Changing the same in productive environment could be a hectic task. For the SAP has provided program which could convert the same in productive environment without deleting existing resources. This has been explained in SAP note 629676 – Resource type conversion
It defines clearly the steps involved to create local report for the resource conversion. The source code is available in correction instruction as per your system version. Please create the program in your development, test thoroughly in development & quality and finally use the same in production.