[tialaramex]

This is part of what Europe has Transfer of Undertakings laws for. Transfer of Undertakings laws basically say if an employee was working for X, and then supposedly they're now working for Y, but actually it's the same fucking job, then since those employees are doing the same job that's the same contract, any rules about that contract still apply.

Under Transfer rules, Unions transfer too, unless the "same fucking job" is actually integrated somehow as might happen after a merger, so that the group of employees being represented by the union are just some fraction of an otherwise indistinguishable workforce. So the warehouse situation wouldn't trigger that, but e.g. if Walmart bought Kroger, closed all the Kroger stores, but kept 10% of the workers, moving them to a nearby Walmart as employees of that store, those employees lose their distinct nature and any Kroger union doesn't come with them.