C#

int offset = '9' + '0';
string test = string.Join("", normal.Select(ch => (ch >= 'a' && ch < 'z') ? (char)(ch + 1) : (ch > 'A' && ch <= 'Z') ? (char)(ch - 1) : (ch == 'z') ? (char)'a' : (ch == 'A') ? (char)'Z' : ((char)(Char.IsDigit(ch) ? offset - ch : ch + 1))));

int offset = '9' + '0';
string test = string.Join("", normal.Select(ch => (ch >= 'a' && ch < 'z') ? (char)(ch + 1) : (ch > 'A' && ch <= 'Z') ? (char)(ch - 1) : (ch == 'z') ? (char)'a' : (ch == 'A') ? (char)'Z' : ((char)(Char.IsDigit(ch) ? offset - ch : ch + 1))));

public string Encode(string source)
{
    StringBuilder sb = new StringBuilder();

    int offset = '9' + '0';

    foreach (char ch in source)
    {
        if (Char.IsDigit(ch))
        {
            sb.Append((char) (offset - ch));
            continue;
        }

        if (Char.IsUpper(ch))
        {
            if (ch == 'Z')
            {
                sb.Append('Y');
            }
            else
            {
                sb.Append((char) (ch - 1));
            }

            continue;
        }

        if (ch == 'z')
        {
            sb.Append('a');
        }
        else
        {
            sb.Append((char) (ch + 1));
        }
    }

    return sb.ToString();
}

int offset = '9' + '0';

public char EncodeChar(char ch)
{
    return ch switch
    {
        Char c when Char.IsDigit(ch) =>
            ((char) (offset - ch)),

        Char c when Char.IsUpper(ch) =>
            ch == 'Z' ? 'Y' : ((char)(ch - 1)),

        Char c =>
            ch == 'z' ? 'a' : ((char) (ch + 1))
    };
}

public string Encode3(string source)
{
    StringBuilder sb = new StringBuilder();

    foreach (char ch in source)
    {
        sb.Append(EncodeChar(ch));
    }

    return sb.ToString();
}

private const string transA = "abcdefghijklmnopqurstuvwxyz0123456789";
private const string transB = "bcdefghijklmnopqurstuvwxyza9876543210";

Quote:

τὰ πάντα ῥεῖ καὶ οὐδὲν μένει

void AddTen(int val)
   {
   val += 10;
   }
...
x = 666;
AddTen(x);
Console.WriteLine($"{x} == {666}?");
x = 666;
AddTen(666);
Console.WriteLine($"{x} == {666}?");
x = 666;
Console.WriteLine($"{x} == {666}?");

676 == 666?
666 == 676?
676 == 676?

string outputString = new string(inputString.Select(c => (c >= 'a' && c <= 'z' ?
(char)('a' + (c - 'a' + 1) % 26) : c)).ToArray());