Despite it being chosen over my campaign submission, I really like what they've done in Eberron... Perhaps because in effect and feel it's very similar to my own campaign world of Tazar.

That said, This is how the two will merge for the next campaign:

I am going to use all of the Rules, Races, Classes, Maps, Magic, and Politics described in the Eberron campaign sourcebook.

The world of Tazar now looks and operates like the world of Eberron.

The History and timeline however, will be replaced with the History of Tazar (more on this When the history/timeline is published).

Succinctly stated: Shortly after the magical cataclysm was averted by bringing magical nodes from the poison world, The world of Tazar plunged into choas as the world's old and new magical nodes clashed and vied for dominance.  Over several years the planet undertook drastic physical changes and there were religous and political changes that coincided to cope.  Old cities were destroyed, new ones rose from nothing.  Continents shifted. The Races of tazar (old and new) did what they could to survive the maelstrom.  As magic was much easier to wield now the old Guild system evolved into the new House system, with those having the wherewithall and forsight to keep their heads about them rising to leadership of these houses.   Religeon bent as well.  The gods of old were joined by another ethos that seemed to develop overnight.  Which of these new gods were real, false or just misrepresentations/misinterpretations of the doings of the old gods is a matter for theologic debate, but the result is the same. (in that Eberron religeon exists alongside "classic" Tazar Religeon... More to come, of course)

The chaos and land shifting lasted for nearly ten years, before the magical forces reached an equilibrium and the planet settled with aspects of both Tazar and the Poison world from which the magical essences were taken. (yeah... Eberron reads just like that by the way "This place is just like that island on the poison world."  "This place is just like Bootytown." Etc.)

During that time the Warforged were created (new ret-conned history of the glomelings:  the old glomelings were now just very well crafted golems to protect early settlements on Tazar) initially to help people deal with the chaos, but quickly were turned to use in the struggle for financial, territorial, and political supremacy as the global upheaval started to calm.  And like in the Eberron history, the more that were made, the more sentient they became, until when the final treaties were signed to cement territorial boundaries, the Warforged were granted their freedom and full rights of other sentient creatures.

The setting is now 3-5 years after the treaties were signed, and 50-60 years after the global shifting ended (all of those years filled with varying intensities of war).

So knowledge of past campaigns may be useful, but it won't be necessary, and Knowledge of Eberron will be helpful, but not always verbatim from the books.

Very little of that affects how you will make a character mechanically, but it is important to note that the gods don't have as much direct influence as they used to.  Their Avatars are not as commonplace to be seen any longer (which may be part of the reason for the cementing of the newer ethos alongside them). 

When dealing with religeon, the Classic Tazar and the Eberron philosophies are very similar, so it will be an easy adjust.  People rarely worship one deity to the exclusion of others, but they often have a favored deity.  A cleric or paladin may choose to worship either a Tazar Diety or an Eberron Diety. (though in the latter case he may actually be worshipping a misrepresentation of a Tazar deity anyway).