I have been taking metformin 500mg (immediate release) twice daily for one year?

Yea I'm pretty sure you need a gram of the er to equal 500 twice a day. Here is what is in the prescribing info for the extended release... In randomized trials, patients currently treated with immediate-release metformin were switched to metformin hydrochloride extended-release tablets. Results of this trial suggest that patients receiving immediate-release metformin treatment may be safely switched to metformin hydrochloride extended-release tablets once daily at the same total daily dose, up to 2500 mg once daily. Following a switch from immediate-release metformin to metformin hydrochloride extended-release tablets, glycemic control should be closely monitored and dosage adjustments made accordingly
Get your pharmacist to weigh in on it with your dr.
Keep a good record of your blood sugar daily. If its going up every day, call your dr. Don't wait until its sky high, if you see an increase every day, call.

500mg means 500mg inters your body/day whatever it stays for 1hr or 24hr; the effect must be the same. So you've been taken 1g/day in divided 2 doses, supposed to be 1g in one dose of ER tabs.