Show warning several weeks prior to -replayprotectionactivationtime
Node should use its warning system to display a warning to upgrade several weeks prior to the -replayprotectionactivationtime date, similar to how nodes traditionally warn when they see BIP9 voting on bits they don't recognise. In both cases, the node can reasonably deduce it needs to be updated to avoid forking off, and warn the user about this.
(As an aside, the replay protection mechanism should be removed entirely once the November upgrade is implemented, but even then such a warning mechanism may remain useful for the next upgrade date.)