There are some cases where renaming asset is not ideal–like my previous comment suggests (if there are sequential variants which need to match some other assets of same variant names, for example). In that case, I RMB-ed asset>show in explorer (seems to only work for limited class types, like static meshes)>verified that file was not present as a local file in windows. Closing and restarting (redirectors were fixed and submitted to source control, before restarting) editor allowed me to add new asset.