when i was doing my grade 8, i changed pieces with very little time to go (also got myself another teacher on top of my original one as i was having so much trouble with the prelude and fugue i was doing)
i cant remember what mark i got in grade 8 i dont think it was anything special as i had never really been taught the musicality or understanding of any of the pieces (hence the teacher change) but i do remember feeling alot mroe comfortable with the change of piece and enjoyed it alot more
as to final outcome, although i cant remember what mark i got for grade 8, i went on to do a music degree, DipABRSM and the CT course, but also now working towards a postgraduate degree in porformance and LRSM so it can have been that bad!
good luck !!! and enjoy your new piece