I would say that you need to make sure that:
1: The data remains unchanged after changing the data types
2: The client application keeps working with the modified database.
As for the first point: Since you indicate that it concerns a limited part of the database, i would make a copy of that part of the database, make the necessary changes in the copy and then compare the data in the original database and the copy, either by making a dump of the table data or use a database comparison tool.
I don't know if it is possible on text fields, but you might also want to check if there are indexes defined on the columns (not likely i think, but better safe than sorry). If there are i would rebuild them after the change of data type.
As for the second point, text columns are accessed in a different way then varchar(max) columns on pre-SQL2005 environments, this might provide a problem in the client application. The functions used to access Text columns will also be deprecated in a later version of SQL Server. Also see this topic
on another forum about this subject.