void Main()
{
    Expression<Func<string, int>> expr = s => s.Length;
    expr.Dump();
    
    var visitor = new ParameterReplacementVisitor(
        expr.Parameters[0],
        Expression.Parameter(typeof(string), "x"));
    
    var newExpr = visitor.Visit(expr);
    newExpr.Dump();
}

public class ParameterReplacementVisitor : ExpressionVisitor
{
    private readonly ParameterExpression _oldParam;
    private readonly ParameterExpression _newParam;
    public ParameterReplacementVisitor(ParameterExpression oldParam, ParameterExpression newParam)
    {
        _oldParam = oldParam;
        _newParam = newParam;
    }
    
    protected override Expression VisitParameter(ParameterExpression parameter)
    {
        if (parameter == _oldParam)
            return _newParam;
        return parameter;
    }
    
}