Is the following going to work as expected:
location /v1/users {
rewrite ^/users/(.*)/accounts$ /v1/users/$1/accounts break;
if ($server_name = 'js.mydomain.com') {
set $backend "api.mydomain.com";
}
if ($server_name = 'js-s.mydomain.com') {
set $backend "api-s.mydomain.com";
}
proxy_pass https://$backend;
}