This is a very common concern before thinking for a siebel upgrade.  As per Anjneya "It does depend on the customization of the older application while doing upgrade.Higher the custom application higher the risk.

Business Requirement :  Need to create a multi-value group for Contacts phone numbers. What should your first step in this configuration be?
Choose your answer….

A. never
B. to insert into Siebel EIM (interface) table such as EIM_ACCOUNT
C. to drop tables from the database
D. to insert data into Siebel customer (data) tables such as S_ORG_EXT
Our Linked Group “Siebel Oracle CRM Experts” is a wonderful place to discuss siebel,