Re: DC agent update = need to uninstall first
fine, it wasn't 105% clear to me what you did, which versions etc. so I wrote about my experience.
i just tried to help and so tested with 0275 version and .exe standalone installer file and it required just one reboot.
If you have different behavior, then document it as much as possible (how to reproduce issue), and open ticket on Fortinet's support to get through to developers and get that fixed.
I haven't found an issue in my case, except one expected reboot forced by Windows.