You should be able to add services, but cannot modify your base package if you want to keep it. Keep in mind the only thing you will be able to add are things like the HD Extra Pack, and HBO and such, any channels not included in your current package aside from "Premiums" you will never be able to access without completely changing your base. DNS should remain intact as long as you never ask to have it removed. Just be sure to inform the rep that losing DNS is a dealbreaker on any offers they have. Your best bet is to have loyalty discounts added if your bill is getting high. Equipment can be upgraded without modifying your base package.
And all of this is up to customer service, what they are capable of doing is not always the same as what they will or are allowed to do.