Blimey. If it's still ATOS then from a customer point of view it's as bad as when they tried to change to the Fares View version that was quickly binned.
Still don't understand why a new account is needed if it's the same people running it.
The new booking engine is (I believe) Vix / SilverRail (I am happy to be corrected!). The old booking engine was Worldline (Atos spun off Worldline as a separate entity some years ago).
Which TIS supplier provides it has no bearing on the legal entity that you are entering into a contract with. In both cases it's Virgin Trains East Coast (or whatever their correct legal name is).
As to why your login details cannot be transferred, the answer is probably that they are hashed (they should be anyway). Hashing is a one way process - in other words you can't reverse it. So it isn't possible to pass on your logon (well, password) details from one technology system to another; the user has to re-enter them.
As to whether your order history (and any other historical data) are transferred from one TIS system to another, that's more a matter for how the databases are structured in each, and whether these are compatible / it's worth the effort.
When the East Coast franchise / directly operated service was replaced by the Virgin Trains East Coast Service, that was of course a change of legal entity that you, as a customer, were contracting with, and permission should have been sought to transfer your data from one to the other.