You aren't supposed to be able to grow sugar, fruit, flowers using farm or irrigated farm: the crop is grayed out in the crop selection window. However, by placing a greenhouse (in planning mode), selecting sugar, fruit, and/or flowers, and then downgrading the greenhouse to an irrigated farm, you can grow these crops on an irrigated farm.
This also appears to work fine albeit a bit slowly; I've been successfully providing fruit to my settlement using irrigated farms for a couple decades before I realized it's not supposed to be possible.
I suppose there's two solutions, either just allow it, or make upgrade/downgrade check if the selected crops are still allowed at the new tier.