Objective-C to Swift conversion related issues

Using Swift
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  1. Have a clean state in source control
  2. Make one change
  3. See what's been change in source control (I'm using source tree, it'll show differences in all files including xib's)
  4. Once you figured what's been changed and how - open your xib's in some text editor (e.g. BBEdit) and do the relevant changes manually in other files.

Or just do it one by one as you currently doing, unless you are talking about 100's of files.