If we change our dates we should be getting 2 $300 gift cards correct? If your dates fall in the time frames and meet all the limitations yes.
For two rooms @ POP? 1 gift card for each package
Can I use that towards the tickets into the parks? -- Not really, but kinda. You have to purchase tickets minimum of 5 days to get the deal. You could purchase 5 night tickets and then upgrade after arriving at guest services at any parks (or add on hoppers or WP&M options) and pay for the upgrades with your gift card though.
I'd also like to do a package dealing with the dining plan. How can I make this work? I'm not sure what you mean. You can add the dining plan to your package reservation and pay for it when you pay for everything else. You can NOT use the gift card to pay for the dining plan portion of the package b/c the package must be paid before arrival and you will not get the gift cards until you arrive and check in.
(the gift cards are better than the PIN code at values correct?) Yes the current pin codes are for about 20% off. $300 divided by 5 nights (minimum stay) is about $60 a night 'savings'. Even on the highest peak nights that more savings than you would get with a 20% pin in a value.