When:
(1) One or more new states are added, or (2) The law fixing the size of the House is amended/replaced.
I wouldn’t be surprised if those happen together, so that adding the new state adds on rather than redistributes seats in the House initially.
Note that D.C. being added (with no change to the fixed size of the House) as a state would actually shrink the electoral college by one, since it would add two Senators and zero reps, but D.C. already has three electors; any other state being added (with no change to the fixed size of the House) would add two electors for the two additional Senators added.
Now you've got me genuinely curious as to whether launching e.g. FiveForty would be considered trademark infringement by the courts.
My hunch is it would still be infringement, since it's similar enough. In the same way Pepsi wouldn't be allowed to launch a cola called Coky.
Also a shame he can't take the FiveThirtyEight trademark with him. He can take his models, but not the brand. Still, his own name "Nate Silver" seems to have just as much recognition as "FiveThirtyEight" so he'll do just fine.