Small function and level of abstraction - clean code - refactor further
First of all, each condition and formatting will be moved to your own class. Let's create a common interface:

interface FormattingRemover
{
    bool Match(string value);
    string RemoveFormat(string value);
}


Now we can use it to abstract the formatting:

public class RemoveBold : FormattingRemover
{
    public bool Match(string value) {
        // your condition here
        return true;
    }

    public string RemoveFormat(string value) {
        return RemoveBold(value);
    }
}


In your class you'll need a list with all implementantions of FormattingRemover, iterate over it and when the value matches, you can remove its formatting.

private removers = AllRemovers();

private string Fix(ChangeLogFixOperationPreferences preferences,string value)
{
    foreach (var remover in removers)
    {
        if (remover.Match(value))
        {
            value = remover.RemoveFormat(value);
        }
    }

    return value;
}


For now on, you just need to add new implementations to provide it with AllRemovers. A good idea is inject it with some DI container, like Ninject.