Recently Microsoft announced that SPFx will have public preview versions. To help developers tests their projects using these preview versions, we should consider adding support for upgrading SPFx projects to preview versions.

I propose that we implement it as follows:

• we use the preview version as the last version in the chain you can upgrade to (eg. 1.10.0, 1.11.0, 1.12.0, 1.12.1-preview.1)
• we introduce a new switch named --preview that you have to use to upgrade your project to a preview version. Without this switch, we will provide instructions to upgrade your project to the latest stable version of SPFx
• when Microsoft introduces a new preview version (eg. 1.12.1-preview.2), we'll replace the previous steps to upgrade to preview.1 with steps to upgrade to preview.2 (eg. 1.10.0 > 1.11.0 > 1.12.0 > 1.12.1.preview.2). We won't support upgrading projects on 1.12.1-preview.1 to 1.12.1-preview.2
• when Microsoft ships the preview version as a stable release, we'll replace the upgrade steps for the preview version with the steps to upgrade to the stable version (eg. 1.10.0 > 1.11.0 > 1.12.0 > 1.12.1). We won't support upgrading preview projects to stable versions

Thoughts?