Yes to both questions. Just do any upgrades anytime before the close of business on the last day of your ticket. Any ticket booth or any Guest Relations (inside or outside a park, or at Disney Springs) should be able to do the process.
Gate price for a 5 day park hopper is $435.59, including tax. $451.56 is the gate price of a 6 day park hopper, also including tax. Math tells me that upgrade should cost $15.97.
If you want to upgrade to DVC APs, bring your DVC card and photo ID for all adults age 18+ (who should also be present at the time of the upgrade). Do the math on what it should cost, including tax, and have that number in mind when you go to upgrade.