For adjustments, you probably want to practice trying different volumes, guessing in advance where it should be for a hymn, then adjusting slowly.

• Do it hopefully gradually enough not to startle the congregation. If there is a big sudden change, they notice that instead of the message in the hymn. :)
• But oh well, carry on & stay with the conductor's beat the best you can and don't stop in the middle! Keep breathing and everyone else probably will too.