I am currently involved in deployment a distributed system, with a highly restrictive radio link which I am not allowed to send VTScada data across.

Modbus communications are in place for this project.

So I have:

CentralVT and CentralPLC at the central office. CentralVT talks only to CentralPLC, which servers as a data concentrator for all remote sites.

Site1VT and Site1PLC. Site1VT is a seperate VTScada installation running on a computer at Site 1, that only talks to Site1PLC.

Note that because CentralPLC and Site1PLC talk to each other over radio, they have different IP addresses, which is my issue.

Then repeat for multiple sites.

Now the operators want to see the same graphics, so I have designated the CentralVT as my master VTScada install that will have all changes made, then the VTScada install at each site will be deployed from CentralVT.

And because of the data concentrator, the data is at the same address no matter what PLC is being talked to. So TankLevel1 is at modbus address 40001, no matter what PLC I am talking to.

But the IP address of the PLC is different at each site.

Is there a way to automatically change the Port tag, or change the IP address in the port tag, based on which computer VTScada is running on?

I can make this work with manually updating the PLC details everytime I deploy, but I would really prefer something automatic to make updating remote sites easier.

There are a lot of ways to handle this. You could use an expression in the Address which runs on startup, checks the machine name, and sets the value of the IP address based on the machine name. This is close to your requested solution but might not actually be the easiest things for you to do.

Since you have a central data concentrator, I assume you connect to each station from that application. You can create single tag type to represent the sites. Each site can have the IP address defined as a unique property of the type. It is also important that each site have a unique area.

From there you can use that app at any site and assign a tag area filter for specific workstations to only load the tags associated with one site. This likely makes updates the easiest but be sure to watch the server lists (if you have any) on the app to ensure that the sites are not configured to be clients only.

To get started, right click on the address to create a parameter expression and check the following from the help files:
https://www.vtscada.com/help/Content/Scripting/Expressions/ExpressionIntro.htm

Check this help info on IfElse() to learn about your options here: https://www.vtscada.com/help/Content/Pro_Functions/IfElse.htm