Having moved to Southland around 10 years ago, I've seen the ongoing denialism of Diary Farmers and Councils that this is/will/must happen. It is near on impossible to have a discussion about it, as you simply get dismissed, usually with comments in the form "what would you know, we've been farming like this all our lives, we know what's best". Unfortunately, nothing could be further from the truth.
The Southland District and Regional councils have been heavily weighted with dairy/sheep farmers, even though farmers make up only a small percent of the population. Their voice, at the moment is simply too loud, I'm not sure how it got so loud (money talks?), but it is very hard to tone it down, so others can be heard.
Having grown up in a different place, but on/around farming (crop based), I believe it would not take all that much to switch from Dairy to plant based farming. After all, Dairy and Sheep farming requires growing grass/swede etc., much of which is reseeded/grown each year.
The biggest issue is the will and re-education of the farming community and building infrastructure and exports to support the switch. Unfortunately, many in the farming community are not well-educated (source: Census) in anything other than the farming practices that they have learnt "on the job".
Many rely on Frontera or fertilizer companies to tell them how to best manage their farms, and most just reject whatever the Government tries to impose, even though it is typically for the betterment of all.
Personally, I think it will happen quickly. I suspect there will be a sudden collapse or forced change that will be devastating for many. Frontera, with its export strengths, may be able to lead the way to a more progressive/stepped change, but they need to get moving, and soon.
There are some innovators now, but without significant infrastructure, they struggle to get a foot hold. Many don't know that Oats were grown in quite a large way many years ago in Southland, before Sheep then Dairy took over - all driven by the almighty dollar. So when the value of milk drops (as it is right now), and another crop becomes more profitable, or perhaps becomes more acceptable in our current climate change - we will see the change that some of us are hoping for...