Your saves from v1.4 are NOT compatible. Your hats and settings should be fine, though.

New Stuff:
-Paintings got reworked a bit. 25 new paintings were added. Some paintings only spawn in specific levels. It's much easier to add new paintings now.
-Added 3 new secret hats. Added a new way to unlock the monthly secret hats.
-Added a sound to jumppads and for when bosses get mad.
-Added a new secret mechanic to the game. (hint: try not murdering stuff this time) (also: you can disable this mechanic via console commands.)

Smaller Changes:
-Fixed Pepper Boss not losing rage effects upon death, again.
-Certain scaling effects can now also change the hitboxes for actors. (Large/small objects should now have more accurate hitboxes.)
-Reduced performance by 90%.
-Nerfed bonus points given via enabling the Enemy HP Multiplier.
-You can no longer change the new difficulty modifiers via menus/console commands, without enabling cheats.
-Fixed rare cases where you could pick up specific items but not get any of it's associated score points due to differences in hitboxes.
-Changed the demo endscreen.
-Added some new obituaries.
-You can no longer carry trash bags across levels.
-Fixed the boss hp bar fade CVAR not being ON by default.
-The game now knows if you are playing on the demo version or not.

Well, this update's a little overdue at this point, wouldn't you agree?

The last update to this game was in March, 5 months ago!
But enough about that. I've made a lot of changes to the game recently, I think you're going to like a few of em.

Let's go through some of the big changes real quick:
-Added new Side Quests to Level 3. Included with these new quests is a new area to explore as well. This may or may not include some kind of fishing minigame. My sources haven't been able to confirm this one yet...
-Replaced the Options Menu with a new one, which includes new tooltips to explain what every option does, and (hopefully) less confusion!
-Added the 2 remaining hats for page 1 of the Hat Menu. Now every hat on page 1 is unlockable! (Also added 2 new secret hats as well.)
-Added new difficulty options/modifiers to the game. You now have finer control over how easy/difficult your Box Partying will be. (Choosing modifiers that make the game easier will cause your score to drain over time.)

And now here's the rest of the changelog, in no particular order:
-Enabled Discord Rich Presence. (Box Party now requires GZDOOM v4.8.1 or above.)
-Your glowstick now changes color everytime you use the altfire.
-Added a new end screen when you beat a level that shows you some stats and whatnot. (Includes special support for multiplayer as well)
-Added that neat little animated transition to some of the menus.
-Replaced one of the fonts with a nicer looking one.
-The game now autosaves upon defeating a boss.
-Added some new visuals to all the onion enemies.
-Added some brightmaps to some textures.
-Replaced some signs in level 3 with a new flat version of the signs that can be put on a wall.
-NPC's can wear more types of hats.
-All enemies have a 1/1000 chance of wearing a hat.
-Improved the Metal Detector quest a bit.
-All "temporary" weapons from Side Quests now use a purple ammo bar to better differentiate them from regular weapons.
-Changed some icons in the hat menu.
-Burn Status Effect now burns faster.
-Burn Status Effect will now quickly dissipate if the subject is submerged in water.
-Enemies now take 25% more lightning damage when submerged in water.
-Reduced particles spawned by certain enemy attacks.
-Pizza Projectiles will no longer cause enemies to bleed.
-THE HAMMER's dashing attack was tweaked a bit to try to prevent them getting stuck on terrain.
-Your health/armor doesn't get reset each level anymore.
-Added inventory bar to the HUD. (Useless, unless a mod adds inventory items) (Imported from another mod I'm working on)
-Fixed some visibility issues with the oxygen meter and score meter.
-Added the libtooltipmenu library, used for the new option menu tooltips.
-Added pink text color.
-Replaced some default keybinds. (Only applies to new players.)
-Replaced the "RESET ALL HATS" script with a new one made in ZScript that's a lot more flexible.
-Added new functionality to BOXHAT to also give "RESET ALL HATS" a list of extra CVARs that should also be reset with the hats.
-Nerfed Bell Pepper's walking flamethrower attack. (Can no longer randomly switch directions while attacking.)
-Pizza projectiles are no longer random, they're instead based on the current rotation of the pizza enemy.
-Added new water explosions (for use with the onion enemies.)
-Added a wet floor sign.
-Inverted the Boss Exhaust bar.
-Optimized the boss code a little bit by making all bosses inherit from a new "BoxBoss" actor.
-Added 4 different types of fish. This may or may not be related to the ever elusive fishing minigame.
-Added new bullet trails to big exploding projectiles.
-Added option to not fade boss's HP bar.
-Added 2 new types of melons.
-Added a new sound for completing a quest.
-Added 2 new secrets to Level 3.
-Added 5 more fun.