Discounts for Existing Customers on Upgrade

[#1]

I currently have Plusnet Unlimited (Contracted) standard broadband, it shows on my Bill as £9.99, there is also a discount showing of £7.50 (Offer Code Discount - juneoffer14usw) from when I joined (essentially £2.50 per month for 12 months). I am interested in upgrading to Unlimited Fibre Broadband and calls - every time I try the upgrade process it shows as £14.99 and no discount (my existing product does show as £9.99 also)
So if I proceed do I lose the existing discount showing of £7.50 (Offer Code Discount - juneoffer14usw) and have to pay the full £14.99 pm or will I get the new 6 month discount taking it down from £14.99 to £3.75 for 6 months - then £14.99 per month thereafter. I understand I'd be signing a new 18 month deal.
Any advice would be good 

[#2]

I would try ringing customer options. I think these are the phone numbers 0800 013 2632 or 0330 123 9197. Be prepared for a wait to get through and they only operate between 09-00 and 20-00 Mon to Frid and 09-00 to17-30 Sat. A moderator will correct these times if I'm wrong I hope. Good luck.