What I said over PM. So everyone can read it.
Typically re-install is never needed. Worse comes to worst, you wipe this one group and re-download headers just for this group.
- I'd set the "Display age" to whatever you want.
- I'd set the "Storage Age" to 1200
- I'd right click the group "download all headers".
When it's done right click the group and "Download Latest".
If it doesn't download much, it's done. If it still is downloading a bunch, that means the "download all" failed. That's OK though because "Download Latest" will start where it failed and continue to fill in the header.
Newsbin downloads headers oldest to newest (that's how usenet works). When you set "Download age" to 30, that's you telling Newsbin "I only need 30 days worth". If you want 500 days, set "download age" to 500, right click the group "Post storage/Delete Stored Posts" then "Download latest". Repeat "Download Latest" till you get them all.
In 6.21 and 6.3, you can tell Newsbin to use "Download Age" again from the group right click menu. "Post Storage/Use Download Age".