Undergrad transfers were done through the portal under what was known as the "one time exception". This does away with the "one time" part. Otherwise, I think the mechanism remains "the portal".
The bigger question, that we don't know and won't learn for some time, is how the "athletes can be paid directly" settlement will change things. Does that change the nature of scholarships, the number of scholarships available, the nature of the National Letter of Intent, and on and on. The Big Ten currently requires member schools to guarantee four years of scholarship to any athlete given scholarship in their NLI. Essentially a contract. If the NLI morphs into even more of a contract, can that contract be written to not only bind the school to the student, but also require the student be more tightly bound to the school? THAT could fundamentally change the transfer process and "The portal". Who knows?