The word was changed to bereth in the Etymologies, but was never changed in the texts (cf. Mereth Aderthad and the compound Merethrond)
Sindarin
mereth
noun. feast, feast, [N.] festival
mereth aderthad
proper name. Feast of Reuniting
mereth
noun. feast, festival
mereth
feast
mereth (i vereth) (festival), pl. merith (i merith). Note: In the Etymologies, Tolkien changed the relevant root from _ to
mereth
feast
(i vereth) (festival), pl. merith (i merith). Note: In the Etymologies, Tolkien changed the relevant root from MER to MBER so that mereth became bereth, but names occurring in Tolkien’s narratives (Mereth Aderthad, Merethrond) were not changed, so this revision was never fully implemented.
A feast held by Fingolfin for the Elves of Beleriand, translated as “Feast of Reuniting” (S/113), a combination of mereth “feast” and aderthad “reuniting” (SA/mereth).
Conceptual Development: This name first appeared as N. Mereth Aderthad “Feasts and Games of Reuniting” in Silmarillion drafts from the 1930s (SM/329). In The Etymologies, the noun N. mereth was rejected and replaced by bereth (Ety/MBER), but as pointed out by Christopher Tolkien, the noun was never changed in the narratives, so Tolkien likely reverted this change.