Planning for a long time, propaganda also came out, but it rained that day. What is the best way to change the plan?

If it is an indoor activity, and the rain has little effect on the activity itself, and some participants are present, it is better to proceed as planned. You can add another activity after the activity, telling those who are not present because of the rain that they can come again, and those who have participated in the activity can be invited again and asked to bring people around them. It's better to give these people some rewards.

If the rain completely affects the activity, it is best to inform the participants in advance according to the weather forecast that the activity will be postponed due to the weather or held indoors. These must be clearly informed, and some compensation should be given to those who see the notice (just send a small gift)