This depends upon how many EJB elements have to be converted. For example, if your application has 200+ Entity EJB and 25+ Session EJB, then this is a significant impact. If your application has 5 Entity EJB and one Session EJB, then this is not so significant.
Another factor is how many programmers will be working on the conversion. Seven programmers can get the job completed more efficiently than one programmer.
Everything needs to be tested, so a test strategy, test plan and test cases need to be written. [ January 02, 2009: Message edited by: James Clark ]