Will installing it alter the original game? I have made my own maps and am concerned about them being altered by this expansion. Anyone know?

From what I know it adds a bunch to it but like the previous expansions the maps should work fine plus you can load base/original expansions if the loader is the same as the one i installed before this came out on gog

*mod, not expansion. whatever term used by someone else (just in case if you saw its called expansion, if not im correcting you personally).

HotA on GOG come as separate app. Although they repacks usful also as GOG (offline) setups, i assume this mod-campaign is another GOGs way for Galaxy. Mods as separate games (more like expansions) much useful for Galaxy and Galaxy users - no interfering with base game, no installation required nor changed.