I've tried doing this, but it has invariably failed.  To much force
will be placed on the retainer that is still attached.  There is also
the issue as to whether there is access to remove all the decay with the
bridge in place.
    It may be possible to remove the bridge, have the retainers re-etched
at the lab, fill the cavity, and re-bond the bridge.  But over the years
my long-term success with Maryland bridges has been mediocre, so I
understand wanting to replace it.

Steve