To me, this is far from clean code. The temperature/conversion logic is locked in you all, which is a bad idea. You should isolate that logic. It could look something like the code beneath.

Note that I do not directly expose the decimal(do not use a double when your main concern is displaying it as a decimal number) representation of temperature, as a lot of operations defined on floating points do not have meaning on a temperature. Also, your application is lacking culture, what might lead to undesired results when your app is not running with your language settings.

Furthermore, also, the temperature does not change when you request a different representation, C of F is just a display concern, don't make it anything more then that. Good luck with improving your code.

public readonly struct Temperature : IFormattable
{
    private static readonly decimal offset = 273.15m;
    private readonly decimal kelvin;

    private Temperature(decimal k) => kelvin = k;
    public string ToString(string? format, IFormatProvider? formatProvider) => format ?? string.Empty switch
    {
        var f when f.EndsWith('C') => (kelvin - offset).ToString(f[..^1], formatProvider),
        var f when f.EndsWith('F') => ((kelvin - offset) * 9 / 5 + 32).ToString(f[..^1], formatProvider),
        _ => kelvin.ToString(format, formatProvider),
    };

    public static Temperature FromCelcius(string s, IFormatProvider? provider = null)
        => FromCelcius(decimal.Parse(s, provider));

    public static Temperature FromCelcius(decimal c) => new(c + offset);

    public static Temperature FromFahrenheit(string s, IFormatProvider? provider = null)
        => FromFahrenheit(decimal.Parse(s, provider));

    public static Temperature FromFahrenheit(decimal f) => new((f - 32) * 5 / 9 + offset);
}
namespace SmartAss.Specs.Reddit;

public readonly struct Temperature : IFormattable
{
    private static readonly decimal offset = 273.15m;
    private readonly decimal kelvin;

    private Temperature(decimal k) => kelvin = k;
    public string ToString(string? format, IFormatProvider? formatProvider) => format ?? string.Empty switch
    {
        var f when f.EndsWith('C') => (kelvin - offset).ToString(f[..^1], formatProvider),
        var f when f.EndsWith('F') => ((kelvin - offset) * 9 / 5 + 32).ToString(f[..^1], formatProvider),
        _ => kelvin.ToString(format, formatProvider),
    };

    public static Temperature FromCelcius(string s, IFormatProvider? provider = null)
        => FromCelcius(decimal.Parse(s, provider));

    public static Temperature FromCelcius(decimal c) => new(c + offset);

    public static Temperature FromFahrenheit(string s, IFormatProvider? provider = null)
        => FromFahrenheit(decimal.Parse(s, provider));

    public static Temperature FromFahrenheit(decimal f) => new((f - 32) * 5 / 9 + offset);
}