DP/Flex: why am I cloudy?

[Little Jimmy]

The answer to my question is probably obvious but for some reason ( old age creeping in? ) I'm cloudy about this. Flex is playing 2nd. When the inning starts, Flex stays on the bench with coach informing me that DP is in for Flex. But when players take the field DP actually goes to 3rd with another player taking 2nd. All legal? Something tells me that when Flex sits down and DP is reported that positions don't matter. But I'm having a midday blank right now and I don't have my books with me.