It's possible in some cases. For example, most sites continue to support SSL v3, even though only very old clients do not support TLS v1. So now would be a good

It's possible in some cases. For example, most sites continue to support SSL v3, even though only very old clients do not support TLS v1. So now would be a good time to tell those clients that they need to upgrade. If we don't do it now, SSL v3 might become too insecure for error messages. (Like, for example, SSL v2 is today.) With regards to BEAST, the problem is that a server has no way of telling if a client implements the mitigation technique. The only 100% safe assumption you can make is that the client negotiating TLS 1.0 is vulnerable.