parfuncs = Association @ ParametricNDSolve[
    {
        stock'[t] == stock[t] × growthRate[t],
        stock[2016] == 1,
        growthRate[2016] == gr0,
        WhenEvent[ t > 2018, growthRate[t] -> gr1],
        WhenEvent[ t > 2018, growthRate[t] -> gr2],
        WhenEvent[ t > 2018, growthRate[t] -> gr3],
    },
    { stock, growthRate },
    { t, 2016, 2027 },
    { gr0, gr1, gr2, gr3 },
    DiscreteVariables -> { growthRate },
    InterpolationOrder -> Automatic, (* alternatively: 1, 2, or All *)
    Method -> {
        "EquationSimplification" -> { Automatic, "SimplifySystem" -> False },
        "ParametricSensitivity" -> None,
        "IndexReduction" -> {
            "Pantelides",
            "IndexGoal" -> 1,
            "ConstraintMethod" -> Automatic
        }
    }
];

ReplaceAll[
    parfuncs[stock][gr0,gr1,gr2,gr3],
    {
      gr0 -> 0.01,
      gr1 -> 0.03,
      gr2 -> 0.05,
      gr3 -> 0.01
    }
]