It's not really practical to replace reeds in an accordion. The cost is $450 minimum, nearly a 50% increase in the cost of a Gabbanelli box. A handmade accordion can be found used for $1500 and up.
I discussed a reed change-out with Blake Miller, and he said, "Changing out the reeds in an accordion is like changing the engine in a car. It's not an everyday operation."
@Nedro thank you and I think the gabbanelli sounds good for Zydeco J. Paul Jr. Played gabbanelli accordions on all of his albums and also in live performance.
Gabbanelli does not make a "cajun" accordeon.
Only a Cajun can make a Cajun Accordeon
Gabbinelli makes a (actually they don;'make this accordeon it is made by another company and branded Gabbanelli) 10 button accordeon with an entirely different reed block design,, nothing like a Cajun accordeon (which is a copy of German and Saxon made accordeons)
You can't retune reeds to be in a different key - you will have to replace them.
On most Cajun style accordions, that would be difficult because the reed blocks are permanently attached. It might be possible to remove the individual reeds and put on new ones, but the problem is that reed blocks are sized for the key of the reeds being used. Higher pitched reeds are shorter than lower pitched reeds, so things might not fit properly.
But the good news for you is that reed blocks on a Gabby Cajun King are removable/replaceable. You can open up the accordion, undo a few screws, pop out the old reed blocks and pop in the new ones. Easy peasy. You can buy the replacement blocks from Gabbanelli - they might even take the old ones back in trade, or you might be able to sell them to someone else.