The more likely shift will be from current 3.7 Roadster owners.
This group are already in the £50/£60k zone, and want that performance level.
Current trad Plus 4 owners were either not looking for Roadster performance, or not wanting to spend Roadster money. So I'm not convinced that many trad Plus 4 owners will make that change.
I agree with Dave.
I think Morgan are targeting a new customer and export market (or UK Roadster owner) with the Plus Four particularly with its Automatic and Air Con options and not Trad Plus 4 owners.
I also think that the Plus 6 is aimed at the export market.
If so both Morgans could be even rarer cars in the UK.