>>From the CONSER Cataloging Manual:
31.19.  Changes that require the creation of new records
...
If the physical format in which the serial is issued changes (e.g., from
print version to electronic version), create a separate record for the
new manifestation of the title in accordance with LCRI 21.3B.

And, from LCRI 21.3B:
21.3B.  Serials
        Rule 21.3B1 lists two conditions for making a new entry for a
serial.  Listed below are two other conditions.
        ...
        d) when the physical format in which the serial is issued
changes (not a reproduction or the same serial in another
manifestation, e.g., a braille edition)
                New Zealand national bibliography
                        (Issued only in paper copy through Nov. 1983)
                New Zealand national bibliography (Microfiche)
                        (Issued Dec. 1983-   only in microfiche; not a
                         microfiche edition ofpaper copy issues)

        David Van Hoy, Principal
           Serials Cataloger
             MIT Libraries
               dcvh@mit.
                  edu
                   =