We don't know.
Haldric is first and foremost an experiment, and whether or not it will be good enough to replace Wesnoth 1.x, different enough to coexist or bad enough do drown and die, we can't tell at this point in time.
We can assure you that we do not plan to force Haldric upon you, if you don't like it. If it is replacing Wesnoth 1.x in the future, then only because you think it's better.
No.
We are rewriting the game from scratch, so naturally we explore mechanics we might want to add to the game for you to utilize in UMC, however these new mechanics will not affect the Default Era in any way.
Factions: No. Maps: Possibly.
The Default Era will be recreated and kept like in Wesnoth 1.x, so expect this to stay the same.
However we can not guarantee that we can recreate every map to the full extent, especially those who are more complex than a standard skirmish.
We might create additional maps, but we won't arbitarily remove or change existing maps.
Yes.
Given the new technology, we have to recreate all content from scratch, including campaigns.
This is a lot or work, and we will not be able to ship Haldric with all the existing mainline campaigns.
For the initial release, we plan to include one mainline campaign. More campaigns will be (re)added in later versions.
In addition, there are a few people in high places who are not happy with the quality of Wesnoth's mainline campaigns. Authors might address this by rewriting large portions of the story and/or making use of new gameplay mechanics.
Thus, changes in Singleplayer Content and Gameplay are expected.
No.
Haldric uses a completely different technology under the hood, with very different strength and weaknesses. Thus, the content creation workflow will be built from scratch to play well with the strength of the new technology.
However, we hope to make the modding experience a lot better than it is in Wesnoth 1.x.
Partly.
Wesnoth 1.x has close to 10000 sprites for units and terrain alone. Not to mention all the icons, items, attack sprites. This is an insane amount of artwork and there is no way we would redo all of that in a higher quality, especially because all of these assets are already of very high quality.
So, we will use most of these graphics in Haldric too. Don't expect a major visual upgrade.
What will change is UX/UI. The new technology has a very powerful UI Toolkit, which allows us to approach things in ways Wesnoth 1.x is not capable of. Thus, we are working together with a professional Artist to rework the UI of the game, both visually and functionally.