If locals are avail in a cust market when they sign up they don't have the option to refuse them for a lower package. Packages are discounted $3 only if locals are not avail so once they are, a customer can add them on to get back to the regular base package price.
If locals become avail in a market where a cust has a grandfathered package, D* will want the cust to upgrade to a current package on top of adding locals. So that would be $3 for locals and I believe it's $5 for current package. In the past, at least some csrs still had the ability to add locals to expired packages even if it wasn't technically correct. I don't know if they still can though.
To add to this, Jimbo is correct. Customers in a non local market when they signed on do not HAVE to take locals if/when they become avail as long as they don't change their base package.