Victor,
Right now we use Time Phase in SNP
I don't know what this is. No matter, since you wish to abandon....
It is unclear to me why you would be using SNP as well as ECC MRP, but I suppose you have a good reason. It is generally unwise to plan the same parts using two different systems and two different methods.
The SAP SNP standard tool for replacing one product with another is Product Interchangeability
http://help.sap.com/saphelp_scm700_ehp02/helpdata/EN/45/932899c2c01444b1542ffe8dbd8721/frameset.htm
SAP APO also supports product interchangeability for Optimization, CTM, GATP, and PP/DS; if you are using any of these modules.
There are also tools in ECC to help you with replacing one material with another.
http://help.sap.com/erp2005_ehp_06/helpdata/EN/5b/da1f604caf11d2b437006094b9c9be/frameset.htm
Best Regards,
DB49