From my experience, keeping the old key has no negative effect. The important thing for further updates is importing the new key.
On the other hand, I do not know whether the old key might be compromised now or in the future. In any case, it seems less safe than the new key, due to its inferior encryption.
And I don’t see any further use for the old key either. I will therefore probably delete it soon, if only to not mistake it for the new one again in the future and then possibly delete the wrong key.