There's probably been some change with the original flight, I think, why else would they move you... You can ask them anyway to move to an earlier flight. They don't even need to know you plan to go landside, just tell them you'll be done early in Jakarta and prefer to wait in Singapore over waiting in Jakarta and/or you are more worried about a misconnect, if they even ask.
One way or another, you are fully within your rights to change to an earlier flight out of CGK (whether your original one or even a one before that)