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 temptat

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.