I have an old Mmabo based website which I plan to migrate to Wordrpess
The site ranks well have many competitive terms and the migration would mean that I will have to change the URLs,
I will 301 permanent redirect all old URLs to new location
but will it have any effect on my search engine rankings? will I be able to sustain all the rankings as they already are?
Indeed.
OP, you shouldn’t believe everything you read here regarding SEO, unless the person can back up their claims, too many people around here who don’t know halve as much as they think they know.
In theory, when search spiders encounter a 301, they simply transfer all the rankings and ratings from the old page to the new one. In practice, you might get a slight, short-term drop as the spiders find their way around the new structure, but it shouldn’t be significant - certainly don’t count that as a reason not to migrate.
That’s a bit over the top! Have a look at the video here: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=93633
Google will consider a link to a URL that redirects as a link to the new URL if it looks legitimate (although they reserve the right to not count any redirects that look like cloaking or spamming). A 301 is a 301 regardless of how you get there, so it doesn’t make any sense for Google to only update the link target after it has crawled each page - it knows that your page is being permanently redirected so it should be able to update all the links and juice automatically. I very much doubt it works as you’ve suggested, unless you’ve got some evidence that it does!
That’s quite a drastic statement that hasn’t been bourne out by the multitude of large and well-known sites that have been rebuilt over the years. The only time I have heard of “ranking plummet” is when the migration was done incorrectly.
thanks for the detailed response
there is also the alternative to NOT to migrate 
You will definitely see an impact… at least short term. Once the 301 redirects are in place you will see you’re ranking plummet and traffic reduced.
Imaging you have a URL on your site with 100 external inbound links. You throw up a 301 redirect to a new URL. When Google first discovers the redirect, they will replace your old URL with the new URL in their index and log that it has 1 inbound link. Over the following days, weeks, sometimes a month or more as the recrawl each inbound link, discover the 301 redirect, and transfer credit for the link to the new URL, the new URL will get credit for more and more inbound links. Eventually once all of the inbound links are recrawled after the 301 redirect has been put in place, the new URL will have credit for all 100 inbound links.
However, even once the new URL gets credit for all 100 inbound links to the old URL, the new URL will not be passed ALL of the link juice/PageRank that had previously been passed to the old URL. As has been suspected for a long time… Matt Cutts recently revealed that there is a loss of PR with each 301 redirect… If you know how PageRank is calculated this makes total sense because of the damping (or PR decay) factor that makes up part of the PR formula.
Assume a damping factor of 15%… This means that if a page has a PR of X and Y outbound links then 85% of the page’s PR gets split over the Y outbound links. 15% of the page’s PR just gets thrown away. If you think of a URL that is redirected as a page with ONE outbound link to the target of the redirect… then again assuming a damping factor of 15% it makes sense that if X PR is passed into the the old URL then (0.85*X)/1 = 85% of X PR gets passed to the new URL (the target of the redirect).
But hey… saving 85-90% of the old PR is better than the alternative… NONE!
yes, I think that its about time that I migrated, mambo has been a dead horse for years