Van's runs into this frequently enough that they have a list of the necessary parts.
I did this conversion about a year ago and the 2 difficult parts were:
1) finding a buyer for your extra parts who would take them all at once. Many want piece-parts... ugh.
2) drilling out the tailwheel bulkheads. Some rivets are hidden beneath the skins at the intersections of the stringers/bulkheads.
Overall, it is no big deal to swap from the 6A to the 6. You'll also need to fill the holes where the 6A gear legs and step used to be. Also, no big deal.