[I am NOT writing the entire 071 dialogue for Altertale rn]

So we get to the 071 replacement. I've been going back and forth on this song for quite a while, mainly because the sequence where you learn the story of Gaster plays out completely differently to the corresponding story of Asriel in the original Undertale. Instead of encounters, it's basically a moderately interactive cutscene with NPCs (being the Gaster followers) talking to you about it.
But I've decided to just release this song as is and if it doesn't end up working for the story sequence I'll change it.

I always found the Altertale set of swaps to be interesting, and am disappointed at how poorly they're executed in the original AU.

Swaps:
The Gaster followers swap with the New Home exposition encounters!
New Home swaps with Snowdin Town, and the location is called the Frosty Fortress, including the "coolest" castle in the Underground, where Papyrus of course lives. We go with a more elemental ice theme here, compared to the "snowy forest + cozy village" vibe of the Frosty Forest.
Gaster swaps with Asriel.
Chara remains unswapped, though their story ends up significantly different.

Leitmotifs:
Hotel (Ostinato)
Gaster's Theme
Snowdin
Intelligence Dissonance

This is not Undertale Last Breath Megalovania, nor is it an Enigmatic Encounter Piano Tutorial or a Star and Planet Size Comparison. It is an Altertale AU, which is an AU like Underswap, Storyshift. The Altertale OST contains motifs from Last Breath phase 3, 5, 6 and Endless Breath phase 1.