You add them along side, and you define them as the "modern" standard, and you discourage, but do not forbid, mixing them up. Ideally, there would be no temptation to mix them up in new code, because the new ones would just b better. Including them into old established code (that can't realistically be re-written) is harmless.

When you have good replacements, at least serious developers will use them. Also there are tools to check for compliance.

I don't think, that you can discontinue strcpy and co.