Stormbringer (1st - 4ed) was Chaosium's treatment of the Elric novels. Its a great, fun game, but littered with a fair few saga inaccuracies - some due to the approach taken by the game's author, Ken St Andre, and some due to the changing nature of the Elric saga itself. These editions are based firmly on RQ1 & 2 as the core game system, with tweaks to reflect the Young Kingdoms. It received scant support; the main supplements are: 'Stormbringer Companion', 'Demon Magic', 'White Wolf (ships, etc)', 'The Stealer of Souls' and 'Bane of the Black Sword' (a paired campaign followng part of the Elric saga. Both very good) and 'Rogue Mistress', which I was involved with. Three later supplements were 'Perils of the Young Kingdoms', 'Sorcerers of Pan Tang' and 'Sea Lords of the Purple Towns'. These latter books were produced under the Australian Stormbringer renaissance led by Richard Watts and Mark Morrison. All are great.
In the early 90s, it was decided to restructure Stormbringer and bring t more into line with the saga. Moorcock has written several new novels since the original game, and it was time for a thorough update. Chaosium took the opportunity to revise the old RQ-based system into something better reflecting the heroc nature of the saga, and the result was Elric! (called that because a whole line was anticipated, including Hawkmoon and Corum). The system is sharper, faster, and better fitted to the books. For many, it remains the best incarnation of the BRP system yet. It spawned the supplements you mention: 'Melnibone', 'The Unknown East', 'Fate of Fools', 'Atlas of the YK', 'Sailing of the Seas of Fate' and 'The Bronze Grimoire'.
In 2001 the decision was taken to do a further revision in the wake of the d20 boom, which spawned the d20 'Dragonlords of Melnibone'. Chaosium also decided to rebrand Elric! as 'Stormbringer', even though the system is Elric! (again with tweaks and some new sections). In my view, although the name change is good, the book itself is relatively weak. The layout and organisation of Elric! was superb - by comparison, Stormbringers 5ed is bloated and inelegant, even though the core system is as good as ever.
With the passing of the licence to Mongoose, 'Elric of Melnibone' is a new take on the saga. But much is similar. I've gone back to the source material and taken a long look at its core concepts and tried to figure those into this new game. Just about everything from the previous editions of Stormbringer/Elric will work with the new game. As I was involved with most of the old Stormbringer and Elric supplements, I've tried to impart consistency and compatibility with the new book.
But the old supplements won't be redone by Mongoose. There's no need, and there's new ground that needs to be covered. If you've got these books, hang onto them and use them with the new game, if you wish to. There will be further 'Elric of Melnibone' material - it's being worked on right now - but it will tread new ground.
Loz