How about using IN?
SELECT CASE
WHEN RTRIM(Customer.Source) IN ('CREDITNEW','CREDITOLD','TOC') THEN 'CREDIT'
WHEN RTRIM(Customer.Source) IN ('DEBIT') THEN 'UC DEBIT'
WHEN RTRIM(Customer.Source) IN ('DEBITOLD','DEBITNEW','TLC') THEN 'DEBIT' -- Defined orginally with space, Crystal uses the space version for logo
ELSE 'UNDEFINED' -- Default logo
END AS CompanyName, CompanyId, TermsofUse
FROM Customer
Edit: Formatting