I really recommend the route of buying from dlsite (purchase points and then buy with points), it's definitely worth the $15 in its current state, and would go a long way to support a Dev that doesn't milk their supporters month-by-month like a lot of others.
The free upgrade doubled the H-content, gave us more variety to act 2 (Catacombs), and a whole new possible route of act 3 (Slimes [sometimes you have a choice whether to choose Spiders or Slimes, but sometimes it's based on your 'choice' without knowing which passage you take]).
Notable improvements that I've observed, and this was only what I noticed through 2 full playthroughs in hitting Slimes route each time:
- Arenas
- Normal, Hard, Impossible...
- At initial glance, Hard starts with one teammate down to start (needs to be rescued from being ravaged), Impossible starts with 2 teammates down to start.
- Balance of the game feels more challenging but routes of losing can be recovered from with RNG in potions/items
- Previously, it seemed like too much corruption was unrecoverable from... heading towards an early game over
- Game over seems to happen easier with some major mistakes, giving a steady challenge throughout every Act
- Benefits and Curses have been reworked/appended
- Charmed status
- you will lose control of that/those allies and they will try H-attacking teammates... see below item for Futa too
- Dopplegangers in slimegirl format
- Possibility to hit this in Act 3, or you might get a giant slime-girl boss equivalent to that of Arachnea's level
- Futanari status
- I think this is caused by mushrooms (haven't confirmed yet)... or a possible side effect by drinking the mushroom power-increasing potions(?)
- I think a few new H-animations for this have been added for interactions with some monsters
- If Futa'd, animation set is different than regular Female-on-Female animations.
- 69, rough missionary, doggystyle
- Items & Potions
- lots more variety
- there's even a virginity restoration potion
- ability to refill empty potions from black wellwater which may change to a random potion over time.
- Masturbation mechanics overhaul
- Seems to happen more frequently with enough corruption/miasma pollution, and the length of the animation seems like it might be variable - previously it seemed like straight orgasms would happen more frequently... now it seems more like 50/50.
- Being Futa'd changes the animation as expected
- Option to try the fight again if lost
- Really necessary if you make the mistake of not taking out dual-reaper spawning coffins early enough!
- Allowing 10 reapers to spawn = game over
- Side note: Reapers seem to cause Charmed effect for sure... not sure if it can be caused elsewhere
- Pregnancy status & Birth
- This is caused by sex with slimes... or a possible side effect of drinking stamina-increasing slime potions (confirmed)
- There is an incubation time for this to happen, and there will be hints that it is occurring in the girls' speech
- Weapons - a lot more variety, as well as interesting new spells
There is a lot more I'm missing in mentioning here... but that's just what I remember.