Do you mean the regular dining plan versus the deluxe dining plan? DP just stands for Dining Plan and DDP is just
Disney Dining Plan but both typically reference the middle level that gives you 1TS, 1QS, 1 snack....
If you're talking about the DDP versus DXDP, yes, you can switch if you change your mind. Keep in mind that lowering the price of your package within 45 days (going from DXDP to DDP) will cause you to pay a $50 change fee.