I have added a new category with forums and deleted a forum since I installed mixed seo......Do I just have to hit submit on the Forum URL Management page in the acp to update this? Also do I have to create and replace the .htaccess file.

Also under the Forum URL Management page in the acp I have this in red "The URL cached contains the SEO delimiter and ID.
Consider setting up an original one." I'm not quite sure what this is telling me to do. Any help would be appreciated. TIA

When you add a new forum / category, you need to set a custom url for it (if you a) want to, and b) activated the forum url caching).
But the new forum will still have a default working url based in it's title until you do so. It's most likely the url with delimiter that you are mentioning, the message is just here to tell you that a) this new forum has no cached url yet, and that since you activated forum id removal, you should not set a -fxx bit for it.

If you delete a forum, it's custom url will first stay in cache, but it's not a big deal.

In both case, you do not need to change your .htaccess. Just install / activate the zero dupe to make sure that the proper redirection will occur when necessary

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