I'm slowly working on replacing existing assets with SF Symbols wherever possible in this app I'm working on. In order to maintain a consistent look on iOS 12 and below I'm replacing the existing 1/2x/3x PNGs with a single PDF, by exporting the SVG from the SF Symbols app into Sketch.
By naming the bundled assets the same as the system names, I can use this handy extension to avoid the "if available" dance every time:
At first I wasn't sure if I wanted to update the older assets, but I think I prefer a consistent look regardless of iOS version even if it does mean a bit of extra work. Moving to PDF assets also means less files to update and hopefully a smaller app size.