Use case

When running flutter upgrade it begins to pull the latest update immediately. Asking the user first before downloading may help in case of bandwidth constraints.
Currently a similar functionally is exhibited by running flutter doctor.

Proposal

flutter upgrade should first look for new versions, and then display (if available), the upgrade version along with the version installed in the user's system asking if it would continue (y/N). Additional information, for example, changelog or size of download can be included.

Running something like flutter upgrade -y would immediately download the update skipping the behavior mentioned above.

Consequently in that case the ability to check for updates by running flutter doctor would then have to be removed (or not, since it doesn't quite behave like the proposal).