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;
}