Deleting winning plots is, and will always be, totally optional.
There is a possible attack where an attacker who can co-ordinate N deep from the tip of the chain can try to coerce a winning farmer to re-write a historical transaction block. This attack is much more difficult and thus less of a risk in new consensus and thus we only recommend deleting and re-plotting a plot to farmers with in excess of 1PB of farm size. Anyone smaller than that would be difficult for an attacker to locate and can more safely continue to farm plots that have already won. We plan to have the software automate the process up to and including kicking off a remote plotting process if the current hardware that a farmer or harvester are on is not up to the task of re-plotting.