So if you have the Lucky feat and you aren't quite sure if it's good enough, and roll again, but roll lower, you can still take your first roll... maybe it was enough after all.Lucky: When you roll a 1 on the d20 for an Attack roll, ability check, or saving throw, you can reroll the die and must use the new roll.
There is an odd RAW/RAI issue with the Lucky feat when you have disadvantage...
It doesn't say you reroll anything, just that you roll an additional d20, and can choose which of the d20s is used. RAW, you could have disadvantage and roll 2 die, then spend a Luck point to roll a 3rd die, then presumably choose the highest of those as the one you use. Jeremy Crawford (D&D Rules guy) has said that the intent would be that disadvantage would be applied first, and Luck point would let you choose between what you roll and the lower of the first two. A bit bonkers in that you're being statistically rewarded for being at disadvantage, but he's stated that it plays into the Lucky flavor of the feat well enough that it's not in their interest to errata it and tell people they can't do it that way, but instead is up to each DM how they apply it in their game.Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined. You choose which of the d20s is used for the attack roll, ability check, or saving throw.