StringExtensions.cs

public class StringExtensions
{
    internal static string ReplaceFirstInstanceOf(this string? original, string oldValue, string newValue)
    {
        string result = string.Empty;
        if (!string.IsNullOrEmpty(original))
        {
            int index = original.IndexOf(oldValue);
            result = index < 0 ? original : index == 0 ? string.Concat(newValue, original.AsSpan(oldValue.Length)) :
                string.Concat(original.AsSpan(0, index), newValue, original.AsSpan(index + oldValue.Length));
        }
        return result;
    }
}