I have written extensively on the topic of Ahmadiyya and Takfir, review some of my essays here:

However, I have recently came across some partial-writings of MGA that have been translated into english, see here,, see page 61-62.

Obviously, MGA was writing that ALL muslims who believe in the theory of abrogation are Kafirs.  This is the earliest instance of MGA doing Takfir.