This can be done through emacs, but it requires a little "trick".
There is a program named etags, which is normally used to make a list
of all procedures or functions in a collection of files to permit you
to find the definition of a procedure or function quickly.  However,
another feature of tags lets you replace all occurrences of a string
in a set of files.

First, run etags on all the files you want to replace the string in.
In your case, it sounds like the files named *.mss are the ones you
want.  Thus, type the command

etags *.mss

to the shell.  This will take some time to complete.

Next, go into emacs.  Make sure you are in the same directory that
your files are in.  Now, type

M-x tags-query-replace

(that's Meta-X, which can be typed by holding down the compose key at
the lower left of the keyboard and typing X).  It will prompt you for
the string to replace and the string to replace it with, and then it
will ask you for a tags file, to which you should simply type return.

If you know how to use query-replace, then this will be self-evident.
Otherwise just type ! (exclamation) until the query replace finishes
(! tells query replace to replace all occurrences of a string in a
single buffer, but you have multiple buffers that you want to replace
the string in, so you'll have to type ! several times).

When you're done, just save all your buffers (C-xs) and exit emacs.
You're done.

If you're done with this question, please type 'done' to the OLC
prompt.

Robert Krawitz / OLC Volunteer / SIPB