I did this mod about a year ago. I got the av123 X-LS at a very reasonable rate, far less than a flat pack. Shipping was even less than the flat pack. The inside, the braces were good and the center brace was square not round, like the flat packs. The walls had clued material I refer to as rag wool, but i am not sure what it is in actuality. I pulled that out and scraped it down pretty good. I installed no Rez on all exposed wood, except where the new X-Over was going in. With all the no Rez the x-over was a tight fit so there was no exposed wood.
I used a router and cleared space for the new tweeter leads. The hole did not need to be changed other than that. I kept the original posts. The bonus was that the speakers came with a perfect grill, which i usually leave in place while playing. Overall, with parts and shipping my project was finished at $380.00 for a complete encore.
Overall, it was a great project and my concerns about the x-over layout was easily handles by the photos here and the kindly support of the members. I did some posts here with photos so with a bit of searching you should be able to find them. Good luck.
Scott