GAC problems

Hello. Your help is very much appreciated.
I am trying to achieve the following, with VS2005:

1. After building the  assembly, deploy it in the GAC.
2. The assembly client(s) will reference the GACed version.

3. If I do minor updates to the assembly, I want the clients to be able to pick it up automatically, without needing to make any change.
4. If I do a major update (breaking one) to the assembly, I want the client to have to change the reference to the assembly explicitly, in order to use it.

The problem:

1. How can I achieve  point # 3 ?? (changing the file version doesn’t change a thing, whereas changing the version number, requires the client to explicitly change the reference).

Thank you,

Roy