Things don't change in Texas until the tanks roll in.
Say you have two farmers; one grows cannabis, the other cotton. Because cannabis is easier to grow, the cannabis farmer has more stock to sell, and if demand from the drug market does not absorb it all, he'll try to sell it on the cotton market, potentially igniting a price war with the second farmer.
In this scenario where hemp & marijuana are legal, cotton farmers could start growing hemp too if that is more profitable.
Typically the opposite happens, where we import crops (cocoa beans, etc) and make the products here to save money.
The original poster said MJ, which is not hemp. Marijuana is psychoactive and hemp is not, though they are the same species. The economic idea of "replacement" is that one good (Coca-Cola) can be replaced by a comparable good (Pepsi) based on price discrimination or differences in quality. My point is that MJ can displace a comparable good (another narcotic like opium) but it cannot replace corn or cotton as the uses of all these products are different.
Now maybe you mean hemp can replace/displace cotton or corn? I definitely agree hemp can displace cotton but that's because the water requirements, fertilization and effort spent harvesting are so much easier for hemp than cotton.
I am really confused by your assertion that a cannabis farmer is going to sell their MJ "on the cotton market." Maybe you mean that resources that would have been spent purchasing cotton commodities will instead be used to purchase cannabis commodities?
edit: Just re-read the parent comment that spawned this exchange. I get the distinction you are trying to make.
> I definitely agree hemp can displace cotton
That's what I'm saying. Hemp is much cheaper and it's been shut out of the market only because cannabis was and the two cannot easily be distinguished. Once they're both legal, things will change.
http://en.wikipedia.org/wiki/List_of_dry_communities_by_U.S....