How to copy Ubuntu 18.04 on one partition to a separate hard drive safely without messing up grub?

I currently have an SSD and an HDD. I have Ubuntu 16.04 installed on my SSD and Ubuntu 18.04 on my HDD on a partition (e.g., HDD has 1TB backup and 1TB for Ubuntu 18.04).

I want to replace the 16.04 installation with my 18.04 installation, so I want to overwrite everything on my SSD with the partition that 18.04 is installed on. Typically, I just reinstall everything using a fresh Ubuntu install, but I prefer just to copy over the installation to save time…

What is the easiest way to do this? Also, I think the grub is installed on the 16.04, so if I overwrite will I have trouble booting?