Copying a proof almost verbatim

I am writing a math paper which has an electronic supplementary document. In the supplement, I want to put a proof of a lemma. This proof is copied and pasted from my previous paper and I have changed some symbols (eg. H becomes X) to put it in context. Is this allowed? I have written on it:

Proof (proof modified from Proof of Theorem 1 in [paper]).

Blah blah...

Is this plagiarism or not? It is only in the supplementary document (which is published online) and I have cited.