I\'m currently playing around with Sifr on my site. Basically I have some coloured blocks at the top of my page, which will change the stylesheets through javascript, depending
I haven't used sifr myself, but I think your best bet is the Rollback add-on. Then you should be able to effectively reload your sifr'd tags in your javascript by rolling back & then immediately "rolling forward" again by re-running your replacement statements (i.e. basically disabling & re-enabling it).
EDIT: Just noticed at the bottom of this page there's a redraw()
method... might actually be exactly what you're after? (again, sorry for the vagueness, but I've never directly used sifr myself)