Enhancement: add support for upgrading SPFx projects to beta versions of SharePoint Framework

[waldekmastykarz]

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?

[garrytrinder]

Adding @YannickRe feedback from Twitter

Well, I find the approach solid: provide the opportunity to go to preview, but it is intentional with the switch and you lessen your maintenance burden by not keeping intermediate preview version and you reduce risk of people unintentionally upgrading to preview. Thus: 🚀 it 🤣

Source: https://twitter.com/yannickreekmans/status/1380952633331044359?s=21

I agree also 😁

[waldekmastykarz]

I just thought about one more thing: if you explicitly specify the preview version number in the --toVersion option, we shouldn't require the --preview option as you explicitly opted in for upgrading to a preview version. Agreed?

[garrytrinder]

Agreed.