I too, switched recently
Years ago I signed up for the Post Office's "Broadband Premium" then last week I looked at their website and the same service is now called "Broadband Unlimited". Buried in the account section is an option to "upgrade" to the new service and even though it may be identical it still counts as switching so I get the half price introductory offer for 12 months. However, as an existing customer, nothing needs to change at my end. Instead I get to hit one button, sit back, and enjoy my £170 off.
I urge all of you to check your own providers for similar loopholes. Or failing that threaten to leave and get your discount that way.