if i conquer a brush battlefield and change it back to sunken temple, can i still pay 1 energy to draw 1 for sunken temple if my unit is mighty?

No, you cannot.

According to the rules regarding conquering and scoring, you may only score each battlefield once per turn (Rule 465). Because "conquering" is a method of scoring a battlefield (Rule 464.1), you have already used your ability to score that location for the turn by conquering the Brush battlefield.

Even though you are replacing the Brush battlefield with Sunken Temple, this swap does not reset the fact that the location has already been scored this turn. As detailed in RiftJudge FAQ #10106:

• The restored battlefield inherits all current statuses and effects from the battlefield it replaced, including the fact that it has already been scored this turn.
• Since you cannot score the battlefield again, you cannot trigger its "When you conquer" abilities, even if you swap back to it.

Therefore, you cannot draw a card using Sunken Temple's ability in this scenario.