Your sales representative will know for sure. Ask your sales rep.
I believe maintenance covers all versions. As long as you're paying for support, you can install any version(s) of the product that you bought. You're still bound by the license limitations. For example, if your license is for a single 2-CPU server, and you already have Oracle 11 installed on a 2-CPU server, then the license allows you to install Oracle 10 on the same server. It does not allow you to install anything on a different server, unless you un-install Oracle 11 from the original server.
Again, this is just my understanding. Ask your sales rep to get an authoritative answer.