is the company obligated to let retired staff keep this, or indeed any ST benefit - No.
Legally obligated ? I agree no, Morally obligated - absolutely.
They were not done as a cost saving measure, indeed the changes were pretty much cost neutral.
So why unnecessarily alienate those unable to fight back ? That's the action of a bully. Those affected weren't consulted during the 3 years of "secret" negotiations, they were eventually told to " get used to it " . ( BA words, published in the retired staff magazine Touchdown )
Prior to the changes staff that resigned lost all ST,
No they didn't, if accepting Severance S.T. concessions were immediately available, and promised for life, if resigning otherwise and being eligible for a deferred pension, ( Minimum 10 yrs. service ? ) then ST was available again upon eventually becoming a "pensioner" at normal retiring age.
So you don't have to worry then, ST - with benefits undreamed of to those now kicked out when they were still employed pre. ST 2009 - will be available to you for many years yet to come. Well done.