For this list, I'm not putting obvious things like "still male" or irrelevant things like "not two guys and a Darkin" because there's no way any lore change would do that.
Things that are staying the same about Swain (confirmed):
- He's still old. This varies from middle age to actually elderly, but he's still definitely older than Garen or Darius.
- He's still Noxian, and still a leader (or at least has the ambition to become one).
- Birds are still a major part of his theme.
- He still uses magic.
- He is still of above-average intelligence (inferred from his quotes).
- His first name is still Jericho.
- He still has some connection to LeBlanc.
Major things that are different about Swain (confirmed):
- He gets at least some of his power from demons, or maybe one demon in particular (Beatrice?).
- Riot has decided to focus more on "Dictator Warlock" over "Master Strategist".
- His passive is now a soul-collecting mechanic, which works differently than anything else in the game (as in, it's not like Thresh's or Kindred's soul-collecting mechanic). He can use his ultimate when he collects enough souls (confirmed by Reav3).
- He's going to be at least a little more attractive and charismatic than his current incarnation. Think more "dapper old man" over "fanservice".
- He joins his troops while commanding them in battle, rather than sitting in a chair with tactics laid out for them (an improvement IMO).
- He has a sinister number of polygons in his left hand.
- He has around 300 voice lines, with special interactions with Katarina, Riven, Garen, Camille, Jinx, Kalista, and Aurelion Sol (IDK why either). He will probably have more special interactions. This will make him have the most voice lines (previous record holder is Jhin, with around 241). Makes sense for such a plot-heavy character.
- Him betraying Urgot is definitely still canon. Him misusing vastayan blood (others or his own) may or may not be.
- He has graying hair.
Unconfirmed but mentioned stuff (some theories):
- His arm was cut off by Irelia in the Noxian Invasion of Ionia.
- Some say that he'll have a demonic prosthetic that the demon granted to him as part of the pact.
Foolish assumptions based on the above information:
- He's still going to be a drain tank / mage.
- He was still involved in the re-resurrection of Sion.
- Darius still works for, under, or with him.
If there's anything I missed about Swain that's confirmed, please let me know, so I can add it. I may have missed more than I thought.
Right now, new Swain looks cool while being faithful to the original (from what we've heard of him so far).
Only one more important question: Swain Feeney Art when?
EDIT: Connection to LeBlanc was moved from "foolish assumptions" to "staying the same", based on evidence from a source