I'm going to buy a developed website that has some content. However, I prefer to put new - my own content on the website. The content will be related to the same keywords as the current content is. Can this change of content cause a drop in the ranking on Google? The backlinks will stay on place, so I think if the backlinks will remain and the content will stay related, the SERP should not change. Am I right?
Since SERP is based on onpage factors too (like your text for example) it's obvious that your position for certain keywords will fluctuate a bit. Some internal pages will rank higher while other will lost a few positions. After changing the content build / buy quality backlinks and you will be fine.
Change in content will definitely affect the rankings, but it can be worked on and increased gradually.
As usual, I'm agreeing with Zexy. Preserve the URL's and you won't have anything too dramatic changing unless you really botch up the new content. Keyword in title, good content, and good alt image/Heading's/contextual links/outbound links/article length, you'll be fine. More back links to the inner pages & you'll have better rankings without question.