Is it possible to purchase and upgrade from LR4 to LR6?
I can see the option to buy new, but the upgrade option is not available.
If not now, will this be available in the future?
Yes, you can buy from the following link. After clicking "Buy" change from Full to Upgrade; then select your existing LR product from the drop-down menu and the correct pricing will be displayed.
Thanks very much for the advice and link.