Yes, existing texts can be optimized afterwards. However, the process is much more time-consuming, as search intentions, focus keywords, etc. have to be adjusted afterwards. In addition, relevance-increasing keywords and user questions are often missing, which must be determined through extensive research. All these aspects must then be incorporated into the existing text and some passages must be reworded. It is important to note that even if you have written an SEO text, you should practice regular content hygiene in order to keep your content up to date and optimize it.