Ticket is not a block, it's a transaction. Block can have 1 or more transactions. Block having no transactions serve no purpose and usually is discouraged in most block chains due to waste of storage space.
Block chains can change up to maximum depth, which is usually same as the chain length when synchronizing nodes, the hash of the first block is compared when syncing, so it can't change.
When people think block chain can't change, they think it as single block can't change, as it would invalidate the block hash, but block can be replaced with another block in primary chain. The previous block in the primary chain is moved to alternate chain that starts at the same height and other blocks can be chained to it to create longer alternate chain.