I mistakenly made a page which is supposed to be a post. Now I have a lot of content in it. Is there a way to make it as post? I have self-hosted solution, so I can do changes in DB directly if required.
I am unaware of a direct method of converting a page to a post – but what I would practically do is a simple CTRL + A (select all) and CTRL + C (copy). Then create a new post and CTRL + V (paste).
Not a “elegant” solution in form of a programmatic one, but it does the job and is a lot quicker than fidgeting in the database 
yes I tried it but it doesnt properly copies the contents in my case, I have tried with one post and it took me actually more time reformatting the content