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Without entering the correct code first, the code cannot be changed.
Besides that, what you describe (green light, an audible click) indicates that the code is correct and works... except for the door not unlatching.
This indicates not so much a changed code, as a jammed latch.
Try pushing inward on the door, while entering the code to unlock.
Once the code has been entered, try rattling the latch a bit.
At worst, you may need to get some powdered lock lubricant, and blow it into the area of the latch. (Warning: grease lubricants will gum up a safe, do not use grease nor oil lubricants, but a powdered silicone-based lock lubricant that is safe for safes.) This may free up the sticking latch, and allow the door to open properly.
(How old is your safe, by the way?)