If there was a court order stating that you must pay alimony starting one year ago, then you may have to pay the back support. This however, is a a separate issue from the request to modify the current support order that you also listed. If there has been a change in financial circumstances, then spousal support may be modified to accommodate those changes. I would strongly recommend that you consult with an experienced attorney to help you with your case. There are many qualified and experienced attorneys willing to help you on Avvo through the "find a lawyer" feature. Best of luck.
Normally, the increase or decrease only goes back to the date of the request, unless there is an allegation of fraud in your income and expense disclosure. Make sure you stay current and research "fiduciary duty" to your spouse to make sure you do not violate your duty to them.