You can't transfer Thank You points earned from checking accounts. In order to get them worth more towards airfare, you'll need to combine your checking TY account with your Prestige card. 51,000 TYP would be worth $816 towards AA flights or $678 towards another airline's flights. However, note that if you redeem for more than $600 in value from checking TY points only in any calendar year, Citi will send you a 1099 for the value. When you combine TY accounts and then make a redemption, Citi uses your soonest expiring points first, so your checking points will be used before Prestige points (since Prestige points never expire).