Here's the problem with Mossberg Model 500 barrel's and magazine lengths.
The Mossberg barrel has a lug with a finger screw that screws into the end of the magazine tube.
The 5 shot models have the lug attached closer to the rear of the barrel, and the 8 shot models attach farther forward.
Because of this, you MUST buy a barrel to fit whichever length of magazine tube is installed in the receiver.
You CANNOT put a barrel made for a 5 shot gun on an 8 shot receiver, and you can't put a barrel made for a 8 shot gun on an 5 shot receiver.
What you can do, is buy a new longer or shorter magazine tube and barrel, and install them on your gun.
Due to the expense, most people just buy the gun already set up with the magazine capacity they want.
Another problem is, the 8 shot guns are pure defense guns, not sporting, so Mossberg doesn't make much in the way of longer barrels for it.
Last, the Mossberg Models 590 and 590-A1 have a TOTALLY different barrel system.
This system is a close copy of the Remington 870 barrel mounting system.
Barrels will NOT interchange between the Model 500 and the 590 and 590-A1.
So your options are:
Live with what you've got, or buy or trade for a different gun with the barrel and magazine length you want.
Buy a new shorter magazine tube and a longer sporting barrel to work with the new tube.