Please note that this here is no official reply from support but just my personal experience.
I replayed The Longest Journey like 3 months ago on Windows 8.1. without experiencing an issue except one which could be easily fixed with a rather "clumsy" workarround.
The Game used to crash sometimes after playing the FMV? Cutscenes.
That problem was easily solved by just leaving a browser Window open in the background playing some around 24h HD Video on Youtube (most likely my graphic card powered down too low while playing that FMV cutscenes and then the game couldn't properly wake it anymore - and the YT video in the background avoids exactly that powering down). With the Youtube "Fix" i could play the game without a single crash or issue and i completed it.
Windows 8.1. and Windows 10 are more or less similar systems and i just did a 15 minute Test of running The Longest Journey under Windows 10 and didn't experience any issue so my verdict is you should be fine and be able to run the game.
Cheers - and ya it's a nice game - i really enjoyed the recent playthrough and it kinda aged "well"
