The following is guidance on using 2014 character options with 2024 characters. This is not about D&D Beyond implementation, which we're still waiting for official clarification on.

• “The book contains many new or redesigned elements and the versions of things in this book replace versions from older books.” (What’s New in the 2024 Version? [sidebar] - Introduction)
• In other words, if there is a character option that exists in older books, but doesn’t exist in this book, then those older options are still playable under the following rules.
• However, if something does appear in this book, you should use the new version instead to avoid having two versions of the same thing at the same table, whether that be classes, subclasses, species, feats, etc.
• Backgrounds and Species: (Chapter 2)
• When using an older species, ignore the ability score increases and use the ones in your background instead.
• When using an older background, select an Origin feat if the background doesn’t already give a feat and either increase one ability score of your choice by 2 and another by 1 or increase 3 ability scores by 1.
• Subclasses: (Chapter 3)
• All classes at level 3 have “A subclass is a specialization that grants you features at certain [CLASS] levels. For the rest of your career, you gain each of your subclass’s features that are of your [CLASS] level or lower.”
• Cleric Blessed Strikes: “You gain one of the following options of your choice (if you get either option from a Cleric subclass in an older book, use only the option you choose for this feature).”
• Feats
• Feats have four categories: Origin, General, Fighting Style, and Epic Boon. 
• Whenever the book tells you to choose a feat, it might explicitly tell you which category to choose from. For example, when the book tells you to choose an Origin feat, you can’t choose an older feat like Eldritch Adept, because they aren’t tagged as Origin feats. In order to be an Origin feat, it must be explicitly categorized as such.
• General feats, on the other hand, are any feats which do not fall under any of the other categories. This, in effect, makes all older feats part of the General category. There's nothing that just says, "choose a General feat". The Ability Score Improvement feature at level 4 in all classes states: “You gain the Ability Score Improvement feat (see chapter 5) or another feat of your choice for which you qualify. You gain this feature again at [CLASS] levels 8, 12, and 16.” The Epic Boon feature at level 19 uses similar language. In these cases, if you qualify for older feats, you can take them at the levels that you gain this feature.
• Spells
• The book doesn’t provide explicit guidance anywhere. That’s because older spells like Booming Blade, Green-Flame Blade, and Tasha’s Mind Whip are playable as is without any conversion.
• Brand New
  • Arcane Vigor, Befuddlement, Elementalism, Divine Smite, Fount of Moonlight, Jallarzi’s Storm of Radiance, Power Word Fortify, Shining Smite, Sorcerous Burst, Starry Wisp, Tasha’s Bubbling Cauldron, Yolande’s Regal Presence
• Replaced. If you want to use the older version, you can. These have different names.
  • Branding Smite (replaced by Shining Smite)
  • Feeblemind (replaced by Befuddlement)
• From EEPC
  • Ice Knife, Thunderclap, Vitriolic Sphere
• From Xanathar’s
  • Charm Monster, Mind Spike, Steel Wind Strike, Synaptic Static, Toll the Dead, Word of Radiance
• From Tasha’s
  • Mind Sliver, Summon Aberration, Summon Beast, Summon Celestial, Summon Construct, Summon Elemental, Summon Fey, Summon Fiend, Summon Undead
• From Fizban’s
  • Summon Dragon
• Everything Else (Artificer, Warlock Invocations, etc.)
  • The book doesn’t provide explicit guidance. That’s because they are playable as is without any conversion so no guidance was necessary.
  • There are exceptions. Pact of the Talisman and Circle of the Shepherd are not compatible with the newer classes.

So the way "backwards compatibility works" is:

• The 2024 core rules are backwards compatible with 2014 character options.
• The 2014 core rules are not forwards compatible with 2024 character options. You can make them work, but there is going to be friction as there are some new terms that may be missing in the 2014 core rules. Examples: Emanation, Bloodied, etc.

If you have 2024 characters in your game, you should use the 2024 core rules.

For any character option that doesn't appear in the 2024 PHB and doesn't have any specific guidance, it's basically "use as is" from the older book.

The only subclass I know of that has issues is the Shepherd Druid, due to expectations it has about how Conjure Animals and other summons work (having HD). Jeremy has stated at Gencon that other subclasses which have not seen a revision yet are slated for one in upcoming books, though he did not specify whether they all would.

The College of Spirits should have no issue working with the updated Bard class.

With regard to Hexblade, the only thing that is redundant is the CHA to attacks and damage. Almost all other features are not present in the 2024 base class: armor and shield proficiency, spell list, hexblade's curse, etc.

What was killed was making it a worthwhile dip.

Quote from Charwoman_Gene >>

Quote from Sillvva >>

With regard to Hexblade, the only thing that is redundant is the CHA to attacks and damage. Almost all other features are not present in the 2024 base class: armor and shield proficiency, spell list, hexblade's curse, etc.

What was killed was making it a worthwhile dip.

 

You misspelled subclass.

 I said class. And I meant class. 

Jeremy acknowledged during Gen Con that they have plans for the Artificer. I think it's unlikely to be in the DMG.