you can't rename a model... you can easily copy a model, but not rename.
even if you went into the xml file to change it -- it would break the connection in the sequence xml files that refer to that name.
you can change the model type, open the xseq file and resave.
still, i have gotten uncomfortable about changing models because the way that nutcracker writes to the existing files -- part of the way that it deals with lsp and lor merges.
so i do one of 2 things when changing models -- changing channels, removing strings, etc.
1) delete the xseq file and open the music file, then pull in the xml file
or
2) open the xseq file -- and while xlights is open go to an explorer window and delete the xseq file -- and resave in xlights. the contents of the old xseq won't interfere because they were removed.