My guess (having lived both sides of the line in USA where you are served grits or potatoes automatically in breakfast at a diner) is that the replacement would work. Two other notes. The whole grits will likely have a different mouthfeel than the instant grits...more of the original nubs of ground (corn, if that's what they are) grits. The instant would have been smoother, like an instant mashed potato. And they may need more water to cook than the instant. Have a look at this recipe from Bob's Red Mill (a reliable source) on how to cook grits and compare it with the recipe you are planning to make. https://www.bobsredmill.com/recipes/how-to-make/basic-recipe-for-quick-grits/
PS Read up more on instant grits after seeing your question. Many users find it disappointing in texture and taste; maybe you are getting better results with regular grits.
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Two other notes.
The whole grits will likely have a different mouthfeel than the instant grits...more of the original nubs of ground (corn, if that's what they are) grits. The instant would have been smoother, like an instant mashed potato.
And they may need more water to cook than the instant.
Have a look at this recipe from Bob's Red Mill (a reliable source) on how to cook grits and compare it with the recipe you are planning to make.
https://www.bobsredmill.com/recipes/how-to-make/basic-recipe-for-quick-grits/