A snapshot always contains the full state of the index at the point in time when the snapshot was taken. Storage-wise they might be incremental by reuse of data from older snapshots in the same repository, but whether it actually is incremental depends on the merging of the segment files.
If the older snapshots are deleted the physical segments files won't be deleted if they're referenced by newer snapshots, i.e. you'll never "break" new snapshots be deleting older ones but it's also possible that you won't reclaim any disk space with the deletions.