The old implementation for Panorma render was added quite hackish. It used the "X Parts" value to extend an Image size horizontally, and rotating the camera for each X Part.
Now the internal render system has become fully tile-based; enabling the Panorama option will automatically apply the panorama effect on all vertically aligned tiles (slices).
So you can just enable or disable the "Pano" button, to get a subtle lens effect, the output image size will still be same as without Panorama.
For Panorama render, the minimum slice size has been hardcoded to be 8 pixels. The "X Parts" button goes up to 512 to allow that. In practice, rendering 64 slices will already give very good images for a wide angle lens of 90 degrees, the curvature of straight lines then is equal to a circle of 256 points.
Rendering a full 360 degree panorama you do by creating an extreme wide angle camera. The theory says camera-lens 5 should do 360 degrees, but for some reason my tests reveil it's 5.1... there's a rounding error somewhere, or something related to the clipping plane start? We will look at that later.
Also note that for each Xpart slice, the entire database needs to be rotated around camera to correct for the panorama effect, on huge scenes that might give some overhead.
Threaded render goes fine for Panorama too, but it can only render the parts within a slice in parallel (the Y-Parts). For the next panorama slice it has to wait for all threads of the current slice to be ready.
On reading old files, the settings from old files are converted to match as closely as possible the new situation.
Currently the code detects for old panorama by checking for the image size. If image width is smaller than height, it assumes it's an old file (only if Panoroma option was set).