nginx replace given character globally in url

Question

I have a legacy backend wich doesn't interpret url-encoded %2B as a + sign in urls like

http://10.20.30.40/keyvalues?Name=foo%2Bbar%2Bbaz%2B...
http://10.20.30.40/keyvalues?Name=foo%2Bbaz
http://10.20.30.40/keyvalues?Name=foo

so I'm going to bring them to the form:

http://10.20.30.40/keyvalues?Name=foo+bar+baz+...
http://10.20.30.40/keyvalues?Name=foo+baz
http://10.20.30.40/keyvalues?Name=foo

with Nginx reverce-proxy. But I'm guess that in my case ngx_http_rewrite_module's config wan't be so simple like in the docs, my current rule is:

    location ~ keyvalues {
        rewrite x2B +;
        proxy_pass "$scheme://$http_host";
    }

and it doesn't replaces (judging by the tcpdump and backend's responces).

What is the correct way to replace multiple characters in the url? (just like with the sed's 's/%2B/+/g' command.)