The term "program transformation" is used in several ways: (a) in the sense of the derivation of efficient programs from high-level specifications; (b) in the sense of optimisations as implemented in compilers; (c) in the sense of software modification as relevant in re-engineering or refactoring. The categories (a) and (b) have always received scientific attention at length. Exploration of category (c) is becoming more urgent in the emerging age of software evolution. Complete foundations and general methods of software modification are needed in order to enjoy the age of software evolution. This age comes with many challenges such as the magnitude of language cocktails, the fast speed at which protocols, APIs, and standards come and go, as well as the necessary complexity and the required flexibility of long-lived, business-critical software assets. In the present talk, I will make a few contributions to the foundations and methods of software modification: (i) grammar engineering, (ii) language-parametric program restructuring, (iii) typed, generic AST traversal. To a large extent, this is joint work with my colleagues here in Amsterdam, at CWI/SIG/VU.