Depends, are you having issues with the windows servers? I’d say if it’s not broke, don’t fix it.
Also consider the implications of changing to Linux, do you have experience with managing Linux servers, or does someone else in the IT department?
You should also remember that some of the busiest websites in the world are running Windows web servers including myspace.com, and microsoft.com.
It’s very trendy to say that Linux is great, because it’s free, but it’s not actually free. Your staff of Windows admins will all need training, or you’ll need to hire an additional staff of Linux admins to manage the web servers. If you do hire Linux admins you won’t be able to reduce your Windows admin head count any as the rest of your servers will still be Windows servers. You’ll also find that Linux admins typically make a little more money than Windows admins (especially as you get more senior admins) as Linux is much more complex to manage that Windows is.