In the case of preprints, it is not not possible change the type, but you can create a new page for the published version and link the two pages (preprint and publication), see "How do I link my preprint to the publication page for the final version?" in https://help.researchgate.net/hc/en-us/articles/14293104044177-Preprints. See, e.g., https://www.researchgate.net/publication/331546436 how this looks like.
My question is whether preprints continue to exist after the information contained in them gets published in a journal. It would be confusing to have two sources for the same information (the preprint and the official publication), so I am assuming that preprints are taken down by the authors after their work gets published in a Journal. My question also is concerned with their citations. If preprints and peer-reviewed articles exist, I don't know if tools such as Google Scholar are adding the total number of the citations received by both papers (I believe they should, as they are practically the same publication, only with different levels of revisions).
Usually, preprints are not deleted after the final version was published. In many cases, the final version is behind a paywall and is not accessible to everyone, but the preprint is. (Although some journals demand to delete the preprint before publication.) Another reason is to document the preprint version that may have been cited already.
Concerning you second question, I am not sure whether Google Scholar combines the citations, but I do not think so. ResearchGate does not combine the two versions into one page, therefore the citations are also not combined.