Can I do that---I got my tix thru
AAA?
Yes. You can take your ticket to the ticket booth and tell them you want to add a day onto it.
However, now that I think of it, there may be a gotcha that makes this cost extra. If you got your tix at a discount - which may be the case when you pre-purchase - either from the Disney site or AAA - when you add the 5th day they may charge you the difference in your discounted price and the gate price.
Example.
If you look on the DLR website (and forget about the adults at kid price promotion) you can pre-purchase a 4 day ticket for $179, but the gate price for this ticket is $209.
You can pre-purchase a 5 day ticket for $189, but the gate price for this ticket is $229.
When you pre-purchase 5 days rather than 4 days, it is an extra $10 (189 instead of 179).
BUT, if you try to upgrade from 4 days to 5 days at the gate, they may make you pay the full 5 day gate price (229). So to upgrade at the gate may cost $50 (229 - 179) per person.
It all depends on their policy and what the value is of your pre-purchased tickets. I know when I got a quote from AAA for a package, they quoted me the full gate price - even though it was a pre-purchase. So you may have already paid the gate price.
To get the bottom line answer, take your ticket to the ticket booth on Sunday and ask them how much it would cost to add a day. They may be able to do this at the GCH concierge. If it is $10 a person and you think it is worth it, then pay for the extra day and you are on your way. If they quote you something higher, then you will have to make an on the spot decision based on the cost they quote you.
I have never done this at DLR, but have done it at WDW. I know DLR does this too, though. I just have not done it there.
One final gotcha. It would be disappointing to do all of the above to get into DL only to have the fireworks cancelled. If the weather is bad, or it is windy, do not do it as the fireworks are in jeopardy of getting cancelled. If the weather looks good, and you pay to upgrade and go into DL, the fireworks may still end up getting cancelled (DL has restrictions based on winds at higher altitudes because of the residential areas). In which case you may be ticked off because the only reason you paid to upgrade was to see the fireworks. That is a risk you would have to take. IMO the risk is acceptable if the weather looks good.