Its up to the DM, but you can look at Detect Balance
The DMG on p. 286 walks us through creating a new race using the Aasimar as an example, and contrasting it to the Tiefling. It has this to say about switching out resistances:
Like tieflings, aasimar have darkvision. Instead of resistance to fire damage, we give them resistance to radiant damage to reflect their celestial nature. However, radiant damage isn't as common as fire damage, so we give them resistance to necrotic damage as well, making them good at facing undead.
This advises you to not switch common resistances against less common ones on a 1:1 basis. It matches the treatment in Detect Balance, a tool to evaluate how balanced racial features are to each other power wise, by assigning a point value for them (which oversimplifies things). This is probably as good as anything to established consensus that you can get.
It groups resistances like this:
- Rare Resistance Radiant, Force, Psychic or Thunder (2 points)
- Medium Resistance Cold, Lightning, Necrotic, or Acid (3 points)
- Common Resistance Fire or Poison (4 points)
- Nonmagical Physical Resistance Slashing or Piercing. (10 points), Bludgeoning (12 points)
If you replace necrotic, you could replace it with at best another medium resistance like cold or acid, that matches the narrative background (depending on the campaign, of course). Because you would not normally get anything out of double resistance, this is already better than what you would get by the rules. Poison or fire, which are more commonly encountered would be a further upgrade over that.
This is different from skills, where the rules tell you that if you get the same skill twice from your class or background, you can pick another one instead (p. 124, PHB).
Note, both radiant snd necrotic would be 5 points instead of 3, for the example given in the DMG, so both solutions by Detect Balance are off by a point. As long as it is that close, a generous DM might be fine to allow poison instead of necrotic.
