If your transmission fluid or filter gets clogged, you may notice the engine revving when you're not accelerating the vehicle — this is called
transmission slipping. If you notice this, take your car in for maintenance immediately, as the damage will only get worse.
Changing the transmission fluid (and filter, if equipped) typically is a maintenance item, and should be done on a regular schedule. On some vehicles, flushing the transmission is part of the maintenance schedule; otherwise, a flush is only needed if debris is detected in the fluid.