Just to dispel the “10,000 SkyMiles is a terrible offer” talk, 5k or 10k is the standard offer for an upgrade or retention. Anything more is extremely rare. The only way to get huge SkyMiles bonuses is by applying for a new card.
So my options are
1) take the 10k skymiles
2) open a paltinum. After a year, transfer as much of the limit to my gold as I can. Close the Platinum and upgrade the gold.
3) open a Platinum and close the gold.
Is there any benefit in not closing my gold?