This is one we hadn’t heard before – that the community number would be different because the policy was under the grandfathering rule. Our initial thought was that while the zone/rate is affected (actual zone and rating zone showing on the dec sheet along with notation about it a grandfathered) the community number would not be affected. To confirm, we reached out to FEMA (FEMA Map Service Center (877.336.2627 or msc.fema.gov/portal/search) which stated the only reason the community number would have changed would be due to annexing of the community (e.g. community originally split and then combined or or vice versa), not grandfathering or a map change. They recommed you contact the county’s flood plan manager to determe if any annex had occurred.
I hope this helps.