There are 6 steps to extend the length of a DMS Datafile.  It is important that you follow these steps.  The "ALLOCATE" option of the DMS:DESCRIBE program will create a .DTA file with no records in it.  If you don't follow these steps and work with a temporary work file, you could end up deleting all records from the "live" datafile.

1.  View the current specifications of the file:
        $ MCR DMS:DESCRIBE STUDENT
        DESCRIBE Option? SHOW

        Number of fields described: 19
        Number of key fields      : 2
        Number of virtual fields  : 0
        Number of field names     : 19
        Structure level           : 2

        Highest position described: 256
        Highest position available: 511

2.  Copy the .DSC file to a new, temporary name:
        $ COPY STUDENT.DSC NEW_STUDENT.DSC

3.  Re-Allocate the new datafile to the appropriate size:
        $ MCR DMS:DESCRIBE NEW_STUDENT
        Type "HELP" for assistance

        DESCRIBE Option? ALLOCATE
        Number of characters in each record (256-2047) <511>? 1000
        Record length adjusted up to 1023 to optimize space
        Maximum number of records? 100

        File: MOE$DKC400:[GREG.TEST]NEW_STUDENT
        100 records, 1023 characters per record.
        The file will be approximately 202 blocks long.
        Actual record maximum: 100
        Proceed <NO>? YES

4  Transfer records from the original file to the temporary file:
        $ MCR DMS:TRANSFER
        TRANSFER V6.8-02 - Information Transfer Program
        Copyright (c) 2000, Campus America, Inc.
        Left-side Data File Name? STUDENT/ALL
        Right-side Data File Name? NEW_STUDENT/EXTEND
        Key Field Load <NO>?
        Number of Level Breaks <0>?
        Command   1? .* TO .*
        Command   2?
        Proceed with the Transfer <NO>? YES

5.  Rename the temporary datafile to the "live" filename:
        $ RENAME NEW_STUDENT.* STUDENT.*

6.  View the new specifications of the file:
        $ MCR DMS:DESCRIBE STUDENT
        DESCRIBE Option? SHOW

        Number of fields described: 19
        Number of key fields      : 2
        Number of virtual fields  : 0
        Number of field names     : 19
        Structure level           : 2

        Highest position described: 256
        Highest position available: 1023