At the upper end, a dumb storage of every single possible position[1] vector-multiplied with every possible ruleset[2] wouldn't technically need to be AIed (at least, not more than the
matchbox AI. Of course, the total moves of a standard chess-game (under the one configuration of rules) isn't trivial, so even if shaving off 'edge' conditions to remove 'possible' game positions with 16,384 white pawns covering a 128x128 board[3].
For every case where "someone changes the rules of Chess and beats the chess machine easily", someone else
could have anticipated those changed rules and created a Artificial-Nonintelligent solution to it (except that the possibilities are (effectively?) boundless). Depending upon what changes are made, it might well be that the current breed of self-training chess AIs (onese that are told "these are the rules and the limitations, go and develop your general strategies") could be fed a changed-rule-chess and do better than a human.
Of course, this assumes that a human hasn't (
despite perhaps their best efforts) induced humanlike preconceptions into the "universal engine" AI.
[1] Covering all
possible possible ones, not just 'standard possible'. Like the consequences of having multiple initial queens per side, boardscapes of arbitrary size and/or geometry.
[2] To cover the potentially very simple (
en passant and castling don't exist, everything else as normal) to the potentially more complex (multi-leap knights, 'huffing' as per draughts) and the potentially weird (pythagorean piece, allowed to make any board-move that is Rank˛+File˛=Diagonal˛ for integer 'Diagonal') or at least possibility-multiplying (promotions for more than pawns, "portable wormhole" rules, dice-rolls and Chance/Community Ches(s)t-style cardplay), and of course whatever's needed to deal with hexagonal/non-Euclidean/other-than-2D boards.
[3] 'Clearly' impossible.
[citation needed] Oh... unless the chess-rules involved have been cross-contaminated with something like 'Reversi', it's black's move and they hold a transmutation power in reserve; all the while allowing the gameplay to have actually gone double-kingless without ending the match!