Check out the cifs.netbios_aliases option (which, to my surprise, is deprecated in 8.1:
cifs.netbios_aliases
Provides a comma-separated list of alternative names
for the node. A user can connect to the node using
any of the listed names.
This command is deprecated.
System administrators are encouraged to write CIFS
NetBIOS aliases to the file /etc/cifs_nbalias.cfg (one
alias per line). Use the "cifs nbalias load" command
to cause the node to process the /etc/cifs_nbalias.cfg
file. For more information, see the CIFS chapter in
the System Administrator's Guide.
So if your original fileserver\share is \\fileserver\share, create a share on your copied volume named the same, which would be available at \\filer\share. Then set a netbios alias on the filer for "fileserver".
This assumes that you're retiring the old fileserver.
Bill