I agree with Amir too. You can't keep changing the tags. This will hurt your SEO efforts. Although tags are not what I consider to be one of the major ranking factors, the search engines do look for consistency. It's ok to update posts in your blog but it's probably not going to help you to change the tags.
You should just create a new page or post for whenever you find you need to change the tags. This strategy will help you to increase your page rank which is a major determining factor for search engine placement.
I recommend everyone check out the Traffic Cipher Special Report. It's a free report from the guys over at SEO Networker. It contains a strategy for getting almost instant first page rankings, and not many people are doing it (or even know about it for that matter). So head over and check it out.
Also, keep an eye on my blog, Online Entrepreneur's Information Station. Morse DeCode Marketing Manual will be completed soon and it will help a lot of people sorting out subjects like Social Media, Video Marketing, and SEO (just to name a few).
To Your Success,