[exim] Stacking or renaming headers

By "Stacking", I mean the common pattern where

Foo: the-ur-foo

is rewritten into

X-Original-Foo: the-ur-foo
Foo: the-no-longer-ur-foo

I am not thinking of a header with addresses here, so Exim's rewrite
mechanism doesn't apply. Is there a "best" or "accepted" way to do
this? In particular, can I do this in an ACL:

add_header = X-Original-Foo: $h_foo:
set acl_m_original_foo = $h_foo:
remove_header = Foo
add_header = Foo: the-no-longer-$acl_m_original_foo

(I am not sure if the last add_header trum^H^H^H^Hoverrides the
preceding remove_header.)

And if not in ACL, can I do something similar in a router or transport?

--
Ian