Sim only upgrade however new number has been advised

I ordered a plan upgrade the other day, currently on a sim only and wanted to increase my data, however emails received today indicate that I have been allocated a new number. As this is an upgrade I thought things would stay the same. Should I just have changed my plan through the app rather than requesting an upgrade? How can I amend this? The correspondence today makes it look as though I’m being treated as a new customer however I’ve been with iD for nearly 5 years. I have tried getting through on live chat this evening but have had no luck, was placed in a queue, initially told wait time would be 10 seconds, 20 minutes later I was still waiting. Please can someone help.