Are there either examples, or a mention of this having happened, of students permanently transferring from one magical school to another in Potterverse?
Canon only please (books/JKR/Pottermore).
Please note that I am only asking about transfers, e.g. you must already be studying in one school. Therefore an example of Malfoys considering sending Draco to Durmstrang is not in scope, since that happened before he attended Hogwarts. Also, guest-studying during Tri-Wizard doesn't count as it wasn't permanent.
Also, if the answer is a "no", please provide evidence (other than "well I read the books before and I don't remember a single case like that", unless you have eidetic memory) from entire canon.