I'm guessing you don't have the original layered PSD file then?
So....
1. Create a Linear gradient using the 'Eyedropper' tool to select the colours from the image you have.
2. However you 'isolated' the border, use it to hide the border with a mask.
3. Use the 'Crop' tool to expand the canvas to the size you want the new border to be....constrain it to keep the right ratio.
4. Place guides around the edges of the new canvas.
5. Use the 'Ellipse' tool to create a new elliptical shape using the guides to get the right size.
6. Fill that shape with the gradient you created earlier.
7. Add a stroke if you think it needs it, otherwise...
8. ...drag this new 'shape' layer below your original.
The idea here is to hide, not remove, the 'old' border and make a new one in its place....behind the rest of it.
Its best to 'hide' the old border so that you still have it to hand....you need it to sample the colours for the gradient but ultimately don't want it as part of the final result.
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Regards.
MrToM.
