You need an amazing test suite, for starters. If the test suite is good enough then even a large refactor isn't that scary.
This was part of the problem for the Segwit2X code: inadequate testing. The original Bitcoin Core codebase is up to a very high standard, and that high standard seems to be the first thing to go anytime someone goes to fork it. This is part of the reason why the developers of Bitcoin Core have so much influence to set future direction of the currency within the Bitcoin community at large; there aren't that many highly-skilled cryptocurrency developers out there. It's unsafe to launch a fork without them.
This was part of the problem for the Segwit2X code: inadequate testing. The original Bitcoin Core codebase is up to a very high standard, and that high standard seems to be the first thing to go anytime someone goes to fork it. This is part of the reason why the developers of Bitcoin Core have so much influence to set future direction of the currency within the Bitcoin community at large; there aren't that many highly-skilled cryptocurrency developers out there. It's unsafe to launch a fork without them.