I'm in the planning phases of a campaign for D&D 5e, but using the Planescape setting. Since this setting is the one which introduced tieflings, I sent the table of potential tiefling traits to my players, and told them that they could replace the tiefling spells/traits with something from that table if they wanted to.
One of them asked if they could change the +1 bonus to Intelligence score that tieflings get by default to be a +1 to a different ability score for their character (I'm assuming Wisdom, since I think they're playing a cleric), and I opted to allow it, so long as they didn't put it in Charisma.1
I am the first to admit that numbers and overall game balance are not my strong points, and while I doubt that this modification is likely to be game-breakingly powerful, it's also extremely possible that I've misunderstood some numerical advantage that I've given this player. I also approved this change with the vague idea that one or both of the tieflings in the group might swap their racial spells for the ones given in the table, but I don't believe either of them is doing so.2
Does allowing a player to move their +1 Intelligence score bonus as granted to their race to another mental stat create a balance issue?
1 In case anyone wanted to know why I allowed this, I already have a (fairly well-earned) reputation with my friends for being the meanest/strictest of our DMs; I try to give lenience when I can. This is a group that leans more Adventure Zone than Critical Role with the rules, which is to say that the rules are frequently thrown directly out the window.
2 Yes, two tieflings. So far, out of four players that have been in contact, we've got two tieflings, one half-elf, and one dhampir.