First of all, I think VDSL2 is unlikely to be a target for a while, probably until full fibre. Openreach will be more focused on removing E-side copper, thus getting rid of POTS and ADSLx, before this.
But there are issues with their current implementation of G.fast that makes getting rid of VDSL2 and moving customers G.fast impossible in most areas:
- Vectoring is intense, so current I believe current port capacity is 48 (96 underway) - compare this to max capacity of 288/344 of the higher capacity VDSL2 cabinets
- They bolted it to the cabinet, so where can they put another pod? (Plus there's the question of vectoring across hardware)
- Current power and fibre is supplied from the existing VDSL2 cabinet - there would need to be new sources
There are others issues I haven't touched on, regarding performance over distance, costs to ISPs to supply new hardware, etc.
However, in the scenario where this did actually happen, ignoring all issues relating to it, I imagine there would be an equivalent package of an equivalent cost to aid progress to move end users over.
My conclusion is that it won't happen and it can't happen under the current amendment (they would really need DP G.fast for it due to capacity). But if it did, then I think there would possibly be a package equivalent to the top tier VDSL2 one (80/20). Even if customers did not want to move, they would have no choice.