The following was compiled by merging the 15 balance updates from this post http://us.battle.net/sc2/en/forum/topic/7979378171 and filling in the gaps from Liquipedia.
For future readers, note that this represents the changes to exiting units from the final patch in Wings of Liberty to the initial release of Heart of the Swarm.
Also, I haven't played every unit, so please leave a comment if I should correct something and I'll edit the post.
Edit - March 19, 2013: Noted that Pylon psionic matrix no longer covers higher ground. Thanks to @chobok for pointing this out. More details here: http://www.teamliquid.net/forum/viewmessage.php?topic_id=278806
- Damage (P-45 Gauss Pistol) - Decreased from 4 + 5 vs. Light to 4.
- Range (P-45 Gauss Pistol) - Increased from 4.5 to 5.
- D-8 Charges (anti-building grenades) - Removed from the game.
- Health - Increased from 50 to 60.
- Speed - Increased from 2.95 to 3.75.
- Nitro Packs - Removed from the game.
- Train Reaper - No longer requires a Tech Lab add-on.
- New ability: Combat Drugs - Heals 2 health every second if the Reaper is not attacked for 10 seconds.
- New ability: Battle Mode - Transforms the Hellion into its Hellbat form (requires upgrade from Tech Lab after Armory).
- Siege Mode - No longer requires upgrade.
- New ability: Ignite Afterburners - Speed boost that increases movement speed and acceleration to 4.25 for 8 seconds. 20 second cooldown.
- 250mm Strike Cannons - Removed from the game.
- The Thor can now switch between two modes: High-Impact Payload and Explosive Payload. The mode shift takes 4 seconds. When in High-Impact Payload mode, the Thor switches to a different anti-air gun (250mm Punisher Cannons) that has 10 range and deals 24 flat damage.
- Thor radius, inner radius, and separation radius increased from 0.8215 to 1.
- Yamato Cannon energy cost decreased from 125 to 100.
- Redesigned ability: Seeker Missile - Can now fire from 10 range. Missile comes out and stays immobile in front of the Raven for 5 seconds while charging up, then rapidly moves (it’s not dodgeable at this point) and explodes at the target for 100 damage + splash. Targeted unit lights up red when targeted. If the unit moves 13 range out of where the Seeker Missile is, the Missile fizzles. Energy cost decreased from 125 to 75.
- The Armory now only has one armor upgrade for both air and ground units.
- Vehicle and Ship Weapon upgrades are still separate.
- Hallucination - No longer requires research.
- Blink - Research time increased from 140 to 170 seconds.
- Range - Increased from 4 to 5 (Anion Pulse-Crystals upgrade still grants +2 range).
- Prismatic Beam - No longer charges up. Cooldown decreased from 0.6 to 0.5. No longer does passive +Massive damage.
- Supply - Increased from 3 to 4.
- New ability: Prismatic Alignment - Increases damage to armored units by 6 for 20 seconds, with a 1 minute cooldown. This does not scale with upgrades. Activating the Prismatic Alignment ability now causes a timer to display over the Void Ray for the duration of the effect.
- Interceptors can now change targets when they are in leash range.
- Vortex - Removed from the game.
- New ability: Time Warp - Creates a temporal field that slows all ground units’ movement speed by 50%.
- Redesigned ability: Mass Recall - Now a simple cast-and-click ability on a player’s Nexus. Once cast, the Mothership and every friendly unit within a 7-range radius will be recalled to the Nexus.
- Psionic matrix no longer covers ground higher than the Pylon. This also means you cannot warp-in or cannon rush onto higher ground.
- Cost - Changed to 150 / 150, from 100 / 250.
- New upgrade: Muscular Augments - Hydralisks move 25% faster off creep (requires Lair).
- Speed - Increased from 3.75 to 4.
- New upgrade: Tissue Regeneration - Health regeneration rate increased from .2734 to 1 health / sec.
- Fungal Growth - Damage decreased from 30 + 10 vs. Armored to 30. Is now a projectile. Speed of the projectile is 12. Range increased to 10.
- Infested Terrans no longer gain weapon and armor upgrades.
- Infested Terran egg health reduced to 70.
- Damage - Changed from 15 + 20 armored to 35 flat damage.
- This unit now completely blocks pathing when placed next to other structures, including other Spine Crawlers. Zerglings and other small units can no longer squeeze in between Spine Crawlers. The scale of this unit has been increased from 0.85 to 0.95.
- Damage - Increased from 15 to 15 + 15 vs. Biological.
- No longer requires an Evolution Chamber.
Overlord / Overseer
- Pneumatized Carapace - No longer requires Lair.
- Burrow - No longer requires Lair.
- No longer spawn instantly, but instead have a three second spawning animation.
- May now be placed on ramps.