Editing using alt-r does not distinguish case

I have a document with many instances of oldword (lc) and Oldword (uc).
I want to change them all to newword and Newword respectively using the conventional Search and Replace.
But trying the usual alt-r oldword to newword <All> replaces Oldword as well.
And trying the usual alt-r Oldword to Newword <All> replaces oldword as well.
I do not want to manage the changes one by one as there are 00s of them. Is there something I can change in Settings or .. ?
Thank you!
Fergus
I find that Search using ctrl-w also does not distinguish case; it would be good if a changed behaviour could be contrived here too.
Any hints?