Yea, we're missing something simple here I think. And a few more explicit details.
For example, what do you mean by:
In the cpanel login, I noticed that the account still has "undefined" as the package
Where, what exactly are you looking at?
I just modified an account, and choose the one option that says No Package.
When I look in List Accounts, Under Package column I see a red icon that says "Unknown" beside it.
Next, I go back to: Upgrade/Downgrade an Account
And select a valid Package.
I go back to List Accounts, and I note the account now has the Package I just gave it, proper.
There is no clear server cache option, changing to another Package should set a new Package properly.