The migration of files in obsolete formats to those expected to survive into the future is a key task in proposed techniques for the preservation of electronic documents. To this end, MyMorph has been developed as a web-based file migration service that allows the bulk conversion of electronic documents to PDF in a manner that minimizes certain aspects of the migration cost. It uses client software running on the user's computer to send files via SOAP to a computer system at the National Library of Medicine called DocMorph, which converts more than fifty different file formats to PDF and returns the results to the user. The MyMorph software has been beta tested since June 2002 by more than 2,000 users who have used it to convert thousands of files to PDF. Nearly all users found the software easy to learn to use, and most report that the conversion is fast. From the user's viewpoint the software minimizes the cost of migration in two ways. First, since MyMorph is freely available, anybody with a Windows-based computer and access to the Internet can use it. Second, the interface permits file migration to proceed in batch mode, requiring minimal user interaction regardless of the number of files converted. This paper describes the architecture of this web-based paradigm for file migration, and summarizes the results of the beta test.