you are correct in what youve read. your ati forend will have spacer adapters with it to adapt to the action tube length so it is not crucial. theyre made in 6 1/2" and 7 5/8" lengths. i would reccomend the 7 5/8" because its easier to fit and work with, and adaptable to a wider variety of forends. the best source for mossberg 500 parts is here
http://www.havlinsales.com/model500.htm. it will swap very easily. its a straight forward dis-assembly and re-assembly. im sure there are youtrube "how to" videos on the process. feel free to give us a call if you have any tech questions and we can talk you through it, Brothers In Arms - (830)-391-7677
on some of the post 2006 made moss 500 guns, some were made with a polymer forend that was actually made for the moss maverick 88. it is a solid polymer forearm with the arms actually molded and pinned into the polymer forend. its a non removable forearm 1 piece unit.
its a nice upgrade to replace that one piece forend with a correct action tube. the one piece mossberg maverick 88 forend that ended up on some of these 500s' is prone to failure and puts a lot of stress on the action arms actuating the bolt.