Positive or Negative Reinforcement for driving user behaviour

Often we find ourselves in a position where we need to drive user behavior in a certain direction without force.

For example let's say we have a new version/design for an app and in order to achieve a smooth landing for its release without having to deal with user rejection.

Instead of forcing it on the user base we suggest them to choose to migrate to it, in this case a new objective arises "all users should migrate to the new platform by a Jan 2019"

To assist this objective we could either go for negative reinforcement techniques e.g.

or positive reinforcement e.g

Given that we're not willing to do both tactics at the same time, which one should we follow?