@Tom Duncan,
First: Make sure you have backups that work.
Second: Go to this website and download the free Disk2VHD tool.
https://docs.microsoft.com/en-us/sysinternals/downloads/disk2vhdThird: On your server, shut down services such as Amazing Charts, SQL Server, and QuickBooks. Any service that writes to a database.
Fourth: Connect a USB drive that has more room than you server's hard drive. At least 500GB more.
Fifth: Run the free tool you downloaded. Choose use VHDX file. Choose use Volume Shadow service. Select your USB drive as the destination and provide a file name such as MYSERVER.VHDX.
Sixth: Wait what could be a few hours for the tool to make an exact replica of your hard disk to a VHDX file on the USB drive.
Seventh: When complete, install Windows Server 2019 Standard on your server. Fully patch it and name it appropriately. Install the Hyper-V role.
Eighth: Make a folder on the C drive of your server called Virtual Machines. In that folder make a new folder called 2016Essentials. Copy the VHDX file from the USB drive to there.
Ninth: In Hyper-V, create a new virtual machine and point it to the previously created folder. Be certain to choose the option of adding a VHD file later.
Tenth: Edit the settings of the new virtual machine. Add processors, RAM, etc. as needed. Add the VHDX file you just copied as the C drive of the virtual machine.
Eleventh: Try starting the new virtual machine. If it comes up, be sure to set IP settings and so forth as any settings it had it will have been lost.
Twelfth: Check the event logs for any errors. Ensure the DFSR and AD event logs give the all clear and that Active Directory is functioning properly.
Notes: Have your installation CD or other media handy. It may be necessary to rebuild the boot partition of the VM using recovery tools if the VM does not boot.
Notes: The above steps do leave out some details. I typed this up from memory while I was waiting for something else to install. It's not like I went to my lab and performed this operation just for the sole purpose of writing these instructions.
Notes: If you've never done this before, DO SO AT YOUR OWN RISK. These instructions are to let you know that what you ask can be done and that I've done it before and to give you an idea of what's involved. These instructions are not the end-all-be-all definitive guide to the cosmos.
Notes: I highly recommend setting the VM up on a test server and making sure it works before wiping your production server. That way, you can always try again as many times as you want. The Disk2VHD tool makes no changes to your server. But once you wipe your server and install Windows Server 2019, you better have things working with the VM as that is your point of no return.
Notes: You may use Essentials as a Hyper-V host by installing Windows Server Essentials from media. But no other role may be on the host other than Hyper-V. You then install full Essentials as a virtual machine.
Notes: I actually recommend that you migrate away from Essentials rather than trying to maintain it.
Notes: I do not support hypervisors other than Hyper-V. If you wish to do the same thing but with VMWare or another Hypervisor, good luck.
JamesNT