Okay I've just purchased a site - it's not properly optimized, it's stuffed with keywords, improper use of Title and other tags, and improper structure BUT it does have some rankings (and I'm happy with the purchase ) Now I want to correct everything. I'd like to do in a manner that rankings are not affected and Google picks up the changes ( and improve rankings ). Please suggest me how should I proceed with the change while maintaining the rankings? If you think changes will cause a drop then how long do you think it should last? I'd also need to redirect URLs since the site currently use '_' in URLs and I've always worked with '-' in URLs. Also, it's a static site as of now and I'd use Wordpress to build a new one. Please suggest!
Sometimes changes can cause major fluctuations in rankings, there is really nothing you can do about it because they are unpredictable. Just change one page at a time so you don't lose all your rankings together. Ranking that go down should come back.
Before you do anything with your purchased website, I strongly suggest that you check out the keywords at Google with the adwords tool to see just how much traffic is actually being generated by the keyword phrases that have been "stuffed" in your html tags, meta tags, etc. If there's not much traffic for them, consider changing what you're trying to optimize and use the tool to find related phrases. As a general rule, when I'm starting a new site, I don't go for the big fish right away, but I'll work on phrases with maybe 10,000 searches per month to 30,000. It's easier to get the #1 slot in this manner.