Actually, you don't care about the meta here, you'll just end up submitting your robots.txt, and Google will do the rest with the datas in the submitted robots.txt.
That's why putting it back to what it was first like will do the trick for mx sitemaps, Google won't fell again on a disallow for "forum" after you submitted.
The sitemaps pages won't be thrown out of index, actually, they are not indexed, just crawled.
Doing this will just delete all those from the Google listing, which is rather an enhancement as it will prevent Google from loosing time spidering again and again useless pages.
So far the robots.txt updates only moved those to the omitted results, and they'll end up being throw out as Google follows robots.txt instructions, but after a lot more time than if you do it at once.
And you don't care losing all those entries, because they are really bad, SIDs in most, meaning no PageRank and duplicate of the good ones.
With your actual PageRank of 4, I think Google will re-index you fully quite fast.
Actually, as said in the release post, a site like yours just went from less than 10 pages in cache to 37 800 one month after thanks to mx Google sitemaps and the phpBB SEO mod Rewrites. This is a fact.
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