I don't understand why you think you need to change your ticket if you want to use Avios to upgrade - and I assume you mean on the long haul flight. All WT+ fare classes are eligible to be upgraded using Avios to Club World. BA2239 (LGW - POS) currently has four Avios seats available in Club World on the 11th July. There's no availability for the MAD - LHR sector on 10th July.

You should be able to ring BA and tell them that you want to Upgrade Using Avios from WT+ to Club World. It should really be that simple.