Migrate a Meteor 2.x application to Meteor 3.x

Meteor 3 removed Fibers. Server-side Mongo APIs are async. The module system enforces strict mode. Client reactivity inside async code needs care. Atmosphere packages often need forking or replacement. Approach the migration in phases. Do not flip the framework version flag first.

Recommended strategy

For 3.6-beta.3 or 3.6-rc.0, prepare retained local data under the previous release before updating.