So SGP/ASCOM doesn't actually hand out ports. Windows does this. And yes, if you plug in the same USB2Serial converter into a different USB port you are VERY likely to get a different port assigned. This is likely the issue you're having. You'll need to go through and set the ports each time you plug things in. Sometimes they'll be the same, other times they won't. You can pull up the device manager and look under ports and it should have them listed. It can be a little difficult to figure out which ports are connected to which thing. The best option is just to plug one thing in at a time and watch for a new port to appear.
This is one of many reasons why Serial needs to die...but it's easy and convenient so it remains in use.
You can go into the Windows Device Manager and set ports but this can be problematic if something decides it wants to assign itself to that port (for some reason). I would generally recommend not assigning the COM IDs yourself.
Hope that helps,