The bits saying "£X a month for X months" in bold is for new customers, what you need to pay attention to is the bit which follows "then £X a month." As that would be price you would pay going into that bundle as a existing customer.
This will recontract you, most likely for 12 months, so you may as well ask for a loyalty discount on top of that (cheaper price for 18 months.)
To make the switch, ring 150 from your VM phone or 0345 454 1111 from any phone -> Option 1 -> Confirm your details -> Option 4 -> Option 5. This option is staffed between 8AM to 8PM M-F, 8AM to 6PM Sa but it is closed Su.
______ Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.