sed '/^abc/{n;/def$/s@def@kelify so handsome@g}' Test

awk '/^abc/{print;getline var;gsub(/def$/, "kelify so handsome", var);print var;next}' Test

awk '/^abc/{print;getline var;gsub(/def$/, "kelify so handsome", var);print var;next}1' Test

awk '/^abc/{getline var;gsub(/def$/, "kelify so handsome", var);print var;next}1' Test