I’d be very careful before upgrading. I raised exactly this issue with iD Mobile because the upgrade offers were worse than what was available to new customers.

I was then given a specific retention offer by an adviser, which I accepted — only for the price and terms to subsequently change. I’ve now spent around 10 days trying to sort it out, with hours wasted on chats, conflicting prices and still no resolution.

I have screenshots of the offer I was given, yet I'm now being told something completely different.

So my advice is screenshot absolutely everything before agreeing to anything. In my experience, the retention offer was enough to persuade me to stay, but what I actually ended up with was completely different.

I have several lines with iD Mobile and have been a loyal customer for years. After this experience, I'm seriously questioning whether staying is worth the hassle.

That's very frustrating and you're right that you would expect to be rewarded for your loyalty not the opposite!