I know other people already suggested writing test, carefully documenting etc so I won't repeat that. Just want to add one more thing: try to prune the code first as such a large codebase is likely to have redundant code built up over time. By doing this you will have some time to understand the code (to know what you can kill and what you can't), and also save yourself some time as the codebase you have to refactor now is now much shorter and probably cleaner.
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