Most likely, you will need a case that was fired from your rifle. Hard to tell how to repair without seeing rest of seater. Can you Post a pic of the seater

A couple big "ifs" : if as you say it is working with no ill effects, I would wait until after the match to effect a replacement. If there are any burrs/sharp edges that could bite you, so to speak, I would ever so gently deburr it. If you have access to a lathe it's no big trick to prep a seasoned case that's been fired a few times in that chamber to fit a breech seater. Biggest hassle is to ream/hone the inside of the neck to offer a nice slip fit for the plunger (while guaranteeing that the bullet is a nice slip fit too) - sloppiness in that regard sort of negates some of the benefits accrued from breech seating. At least that's how I view it.

The breechseater was made by Russ Weber who also provided two extra cases, was mainly wondering about the preferred method of disassembly, if there is one. 
Thanks!
Joe

Thanks everyone,

I decided to make the repair, and so far, so good. There are two little flat marks on the seater base, which I believe are index marks to indicate where the case rim has to be removed, am I correct?
I calculate the thread as 5/16X32, can anyone verify that? I am using one of the extra cases Russ provided, but want to replace it. I think I bought this breechseater in 2008 so I probably wont live long enough to have to replace another case. 
thanks for your help!
Joe

I believe you are correct about the flats.

Before you start does the bullet not now fit? If so and if the mouth of the case still has some room to expand and to still go in chamber. I would just turn a new expanding plug to just do that. What is unusual to me is if Russ Weber made the extra cases for the same bullet being used at the time why would it not now fit. The problem with trying to expand the mouth of the case is it will have some spring back involved and it will try to return back to being close to its original state. Same as it does after the case has been shot. If you have the ability to adjust the case so it close to having zero run out. Often times is better to remove the brass from the outside of the case by use of the proper size pin guage is being inserted into the mouth of the case and to then take it out and to re-form it by re-fire forming it. This could also be easily done buy using a fine cut file while the case is running true. Or to use an abrasive to clean up the mouth of the case until the proper pin guage fits in that case I would probably use 220 grit wet sand paper as it shouldn't take much to reach your actual intended target size.

I provided Russ with three fire formed cases, and have one installed to replace the damaged one that was originally installed, so I am good to go.  I still have one fire formed case that I got from Russ as a replacement. However, since I am involved in the project I wanted to make a new replacement case so that I will still have two extras, which I know is a belt and suspenders approach but I cant help it!
At any rate, I just ordered a 5/15 X 32 tap. Will use a fire formed case that has not been sized, ( I have lots of them), and if need be will either ream it out with a shop roll abrasive wrapped around a mandrel or make an expander die ( which I might have from my days of using fixed ammunition). 
The repair was quite simple, but I just wanted to avoid making a stupid mistake, so I thank everyone for their expert advice!
Joe S