I believe you want
:%s/foo\(\w\+\)Bar/& = \1 + \1\Old/
explanation:
\w\+ finds one or more occurences of a character. The preceeding foo and following Bar ensure that these matched characters are just between a foo and a Bar.
\(...\) stores this characters so that they can be used in the replace part of the substitution.
& copies what was matched
\1 is the string captured in the \(....\) part.