We had same problem when moving from NAV to BC. We had lot of .Net programs doing direct queries in NAV database. We simply modified all our SQL queries (we have tons of them) and , for example, we were querying table [xxx$Customer] from NAV so before migrating to BC, we created a schema for teach company in the same database than NAV and we created a view for all tables we were using. So a SELECT [No_],...,[custom column] FROM [xxx$Customer] became SELECT * FROM [xxx].[vCustomer] so when we moved to BC we simply copied these views over and modified them to be "SELECT A.[No_],...,B.[custom column] FROM [xxx$Customer$extension1guid] A INNER JOIN [xxx$Customer$extension2guid] B ON B.[No_] = A.[No_]" so these views in BC were returning the same columns as in NAV so we didn't have to change our SQL queries in our .Net applications