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For the more complex tasks you may need to use a multi-file text editor of some kind - something with support for regular expressions would be ideal, if you're comfortable with regex.
Some "replace" tasks you might be able to automate easily, and others you'll probably need just to use a Find function and inspect each candidate visually to make sure it's what you want. A lot will depend on the patterns you find in your pages. Nested font tags can be tricky - they may each hold different but important information.