Suppose I publish an article in a journal as open access. As far as I know, I do have the possibility to do with my article what I want. I then would like to republish the article with an identical content but a different layout. What are my options? Where can I upload the article (I know there exists ResearchGate, Arxiv and similar sites)? Does republishing the article affect citation scores, impact factor etc because now there exist two versions of the same article?
As far as I know, I do have the possibility to do with my article what I want.
Actually, depending on your agreement with the journal, you may be quite restricted in what you can do. When you publish an article with a publisher you will generally make some kind of contractual agreement with the publisher. For academic journals, the usual practice is for the journal to require a copyright transfer to the journal prior to publication, though there may be some cases where a lesser agreement (e.g., a publication license from the author) suffices. In this case, the usual legal position is that the publisher obtains copyright in the article, but the author maintains a set of "moral rights" or "authorship rights" relating to their authorship, which are generally preserved by statute. The specifics of what you are and aren't allowed to do depend on your agreement with the journal, and the IP laws in the relevant jurisdictions. Nevertheless, the usual case is that the journal agreement will involve you transferring copyright in the article to the journal.
If you have transferred copyright to the journal then you will generally require their permission to republish the article in another forum. There are some exceptions for this relating to personal use under your "authorship rights" and there are standard copyright exceptions for "fair use", etc. The best way to find out your options here is to first contact the journal where you initially published the article, tell them what you would like to do, and ask them if there is any restriction preventing you from doing that. If the journal holds the copyright then you may need their permission in order to republish the article on another forum. Most journals are okay with publication of pre-prints on academic sites (e.g., research-gate, ArXiv, etc.) but you should check with them.