Assuming you have ID changes figured out, actually upgrading your server without resetting the map requires you to remove some of the old files. I recommending moving them to a backup location in case something goes wrong.
For upgrading any modded server, I would recommend following these steps:
Create a new folder to hold your backup. If you are running a server on your own computer, this folder can be anywhere. If you are using a host, you have two options: (a) Using a FTP client (I recommend FileZilla), create a new folder on the remote server for the backup, or ( Using an FTP client to download the files from the server to your computer.
Copy the /mods, /coremods, /mods, /config, /world, /world_nether, and /world_the_end folders and the server .jar (Voltz.jar) to your backup location. If the server is running on your own computer, you can simply copy and paste the folders. If you are using a host, either use a FTP program to copy them to the folder on the server you created earlier, or use the FTP program to download the files to your computer.
After making your backup, delete the /mods, /coremods, and /config folders, and the server .jar.
Download the latest version of the modpack you wish use. After extracting the zip, you should see three folders (/mods, /coremods, and /config) and one file (in this case, Voltz.jar). If the server is running on your own computer, simply copy and paste the files into the folder with the rest of the server files. If you are using a host, use a FTP program to upload the three folders and the file to your server.
All done! Your server should be upgraded. If anything goes wrong, just delete the files you backed up earlier (/world, /mods, etc.) and copy the old files back into their original location.
Hopefully this is helpful. Feel free to share this with anyone else who needs help.