更新站点（上传.php文件）而不会出错的好习惯？

Question

So, lets imagine a site with high traffic. From time to time its being developed and new files/assets have to be uploaded/updated, while site is still under usage. Purely uploading files with Total Commander is not an option. I know its always a possibility that put a white screen "site is under maintenance" but thats an overkill. If I edited only one file about "sending a post to the forum", its about 2 seconds to upload that code, its not worth to close the whole site. There are features (editing own profile) which not connected to that feature. So somehow while the "sending a post to the forum" is uploaded, site just hangs on for some seconds. Users can never realize anything.