Their is only one tank I know of that has that flange to change bladder like Ron said. The reason I believe it the bladder was not too good in the first place.
Before trying to do this job, be advised that it is probably one of the messiest jobs your ever going to encounter. And it's not as easy as it looks.
Do yourself a favor and buy a new tank. If you want one that will last a long time, go with either Flexcon or Well X Trol.
bob...