I have a route-map which defines the networks to announce to our ISP (using a prefix list). I now want to send a community string to the ISP - adding to this route-map statement. Can I just add the "set community xxxx:yyy" to the route map without removing the neighbor statement which refers to that route-map? Or do you need to remove that neighbor statement, add the line to the route-map, and then add back the neighbor statement?
You can add the set community to the route map and not need to remove the neighbor (which would be pretty drastic). The added statement in the route map would apply to any new advertisements of routes but would not affect routes already advertised. So after you change the route map you will need to clear the neighbor so that routes will be advertised fresh and will have the community set where appropriate. Depending on the version of code that you are running there are various possibilities of soft reset for the neighbor.