I don't think it will be down that long at all, Pickles; the upgrade itself will probably take about half an hour, and then I should be able to turn the forums back on. Then I'll have to fix up the templates and mods, which will probably take me a day or two, during which time you may experience some weirdness. (But hopefully not downtime!)
I advise you to have a supply of balloons and a book in case it takes longer then expected though
