Sometimes MS SQL Replication developers use similar names for similar meaning but which can have very different results. These settings both specify alternate location for the Snapshot files, however, one is set at the publication and one at an individual subscriber.
The alt_snapshot_folder parameter is set at the Publisher for ALL subscribers. This can be set at publication creation or after the publication is generated. When this option is set, a new Snapshot is generated and copied to the alternate Snapshot folder.
sp_changepublication @publication = 'TranUpdate', @property = 'alt_snapshot_folder', @value = 'D:\temp\CHRISSK1$SQL2K8_ADVENTUREWORKSLT_TRANUPDATE', @force_invalidate_snapshot = 1
Msg 20607, Level 16, State 1, Procedure sp_MSreinit_article, Line 99Cannot make the change because a snapshot is already generated. Set @force_invalidate_snapshot to 1 to force the change and invalidate the existing snapshot.
The AltSnapshotFolder setting is made at an individual subscriber level and does not require a reinit. All it requires is you first copy the current Snapshot files to new directory. See Tips to improve performance when applying Snapshot in Transactional Replication