This is how I change people's packages over to a different username in npm. It requires admin powers, of course, unless you're doing it with your own username.

npmswitch-example.log

$ curl http://registry.npmjs.org/-/by-user/testuser
{"testuser":["npm-test-blerg","npm-test-publish"]}

$ curl http://registry.npmjs.org/-/by-user/testuser2
{}

$ npmswitch testuser testuser2
+ npm cache clean npm-test-blerg
+ npm owner add testuser2 npm-test-blerg
npm http GET https://registry.npmjs.org/-/user/org.couchdb.user:testuser2
npm http 304 https://registry.npmjs.org/-/user/org.couchdb.user:testuser2
npm http GET https://registry.npmjs.org/npm-test-blerg
npm http 200 https://registry.npmjs.org/npm-test-blerg
npm http PUT https://registry.npmjs.org/npm-test-blerg/-rev/7-3f9d49f42448eb319fa2581cb5abaed6
npm http 201 https://registry.npmjs.org/npm-test-blerg/-rev/7-3f9d49f42448eb319fa2581cb5abaed6
+ npm owner rm testuser npm-test-blerg
npm http GET https://registry.npmjs.org/npm-test-blerg
npm http 200 https://registry.npmjs.org/npm-test-blerg
npm http PUT https://registry.npmjs.org/npm-test-blerg/-rev/8-a67e81b92157daecccd7a9f66b3a9275
npm http 201 https://registry.npmjs.org/npm-test-blerg/-rev/8-a67e81b92157daecccd7a9f66b3a9275
+ npm cache clean npm-test-publish
+ npm owner add testuser2 npm-test-publish
npm http GET https://registry.npmjs.org/npm-test-publish
npm http 200 https://registry.npmjs.org/npm-test-publish
npm http PUT https://registry.npmjs.org/npm-test-publish/-rev/11-5e0d022e73b8793beedf416ea6ed8a59
npm http 201 https://registry.npmjs.org/npm-test-publish/-rev/11-5e0d022e73b8793beedf416ea6ed8a59
+ npm owner rm testuser npm-test-publish
npm http GET https://registry.npmjs.org/npm-test-publish
npm http 200 https://registry.npmjs.org/npm-test-publish
npm http PUT https://registry.npmjs.org/npm-test-publish/-rev/12-90756085000ffd5dfe9fbe399adb1efd
npm http 201 https://registry.npmjs.org/npm-test-publish/-rev/12-90756085000ffd5dfe9fbe399adb1efd

$ curl http://registry.npmjs.org/-/by-user/testuser
{}

$ curl http://registry.npmjs.org/-/by-user/testuser2
{"testuser2":["npm-test-blerg","npm-test-publish"]}

npmswitch.bash

npmswitch () {
  local n1=$1;
  local n2=$2;
  curl -s http://registry.npmjs.org/-/by-user/$n1 \
  | json $n1 \
  | json -a \
  | xargs -I P sh -x -c '\
    npm cache clean P && \
    npm owner add '$n2' P && \
    npm owner rm '$n1' P \
  '
}

@isaacs the route you use in this gist http://registry.npmjs.org/-/by-user/:user seems to be throwing 503 errors these days. has this route been deprecated/removed? tested with curl http://registry.npmjs.org/-/by-user/isaacs