In my opinion, if the company has been paying the bonus to all eligible staff for a number of years, you may have a case based on "custom and practice". In other words, what happens in practice, regardless of whether it's in the contract, may be deemed to be an entitlement.
You'd need to be sure though that there isn't a document in existence, available to staff, which sets out the circumstances in which the bonus is, or isn't paid.
Perhaps wait to see what happens before deciding whether to take more formal action?