SetEnvIfExpr with response header
Solution 1:
If may be evaluated too early in Apache's processing to use attributes of the response. The documentation isn't clear and I can't find that elsewhere right now.
But the Header directive is processed late, so it can use the response. You just need to add the expr condition to it:
Header add X-test-foobar "foobarnew" "expr=resp('X-foo') == 'bar'"