Can I rewrite sentences after my article has been accepted, when reviewing proofs?
The key word in your question is "some." 

If "some" refers to a small number (less than about 10), and the "rewrites" are relatively minor, this should not be a problem. On the other hand, if you have an extensive list of changes spanning multiple pages, and the changes affect the interpretation and argumentation of the article, then the publisher will likely balk at making all of the changes without obtaining editorial approval