A motion of consent means that you both agree. There is no hearing, you both sign the motion order and file it at the courthouse. It is probably the cheapest way to get an order changed.
If your ex is willing to negotiate then this should be your preferred method. It is a one-page form and you don't have to file financial statements if both agree. The duty counsel and the clerks in the courthouse can help you fill it out.
If you don't agree, then you will have to file a financial statement with your motion application to get the amount changed.
The FRO only enforces court orders. They do not make changes on their own, you cannot just show them your tax return. They are not a decision making body. If you have an existing court order then you must have it changed.