I merged from phpbb3 to mybb a few weeks ago and used google SEO and submitted my sitemap. It's been about 2-3 weeks since i submitted my sitemap On phpbb3 i had a few thousand pages indexed within a couple days but for some reason it won't get higher than 3 now and keeps going back to 0 every few days. my site is pagerank 3 i have 986 "Not Found" errors according to google webmasters. i set up the 404 page redirect thing that came with Google SEO. Does anybody have a clue what's going on?
IMO you should try 301 redirection if have made it the permanent move. It is beneficial from the purpose of SEO instead of 404 because 404 doesn't shows that where that page have been moved. So try this one.
This is pretty common for forum type website. I would recommend you do 301 redirect on each and every 404 pages first. Many of 404 pages appearance could harm your website trust towards Google and caused a dropped in ranking later.
USe social bookmarking sites to get your site indexed faster. I bookmark my each page of teh site in about 20 social bookmarking site and trust me the result is amazing.
Your new pages will naturally get crawled by the Google crawler in the future but it will take some time. Perhaps this is one of the reasons why webmasters are recommended to use the best forums systems before launching them to prevent possible problems in the future.
It's very common when you change something on site, particular in the case of url changes, it may create too many 404 errors, hence google takes too long to index new urls from the begining. IT may be faster if your 301 redirect old urls to you new urls. other wise it's too common.;