Great question. I cannot find a good quote for this.
(We do have the scholion 10.73c "it is clear [Epicurus] states that the cosmoi are also perishable because their parts are transforming")
Impermanence is certainly part of the discussion of whole natures vs. qualities -- with only the necessary qualities of whole natures existing as permanent (starting at 39c).
[Don, I accidently posted just after you, but I see we jumped on the same section!]