I have an old blog (apr. 2 years old) with a PageRank of 3. I stopped posting some months ago, but now I'd like to use it again. It has 554 indexed pages, 222 internal and 35 external links. I don't want to change its niche, but it would be better if I could delete the previous posts. How would it effect my PageRank? Should I just leave the old posts on the site?
If you will delete any page, pr will finish for that page, if you will change content, pr will not be disturb also google will like that but it should be unique, not copy paste.
If you move or delete the posts, the PageRank those pages accrued over time will not pass over, nor will they be able to flow through the rest of your site the way they used to. So go ahead and keep them. Besides, you never know if someone else will find that content useful to them (or find your site via that content in a search and contact you about an advertising deal, a sale, or something else).