I agree there is very little evidence other than in Ammianus, Probus and a letter written in the late 3rd Century to support my view about Centuries and Maniples still being in existance during the 4th Century and beyond. However, absence of proof is not proof of absence as they say. My own view is that because there are mention of their being old style Cohorts, Ala, Equites type units on 4th/5th/6th century AD Papyri and inscriptions from Egypt then this might be circumstantial evidence in favour of my view above. I wait to be proved wrong.
Nice debating about this most interesting of topics.