Because assembly is not actually being used in the current code, it is not a breaking change. Type could be specified same as it is now (ex. type="Name.Space.Bootstrapper") or fully qualified (ex. type="Name.Space.Bootstrapper, Assembly.Name"). The change to the line in question would be discarded. This is how I am currently working around this issue, but with a dummy assembly attribute. 
```
<nancyFx>
  <bootstrapper type="Name.Space.Bootstrapper, Assembly.Name" assembly="thisIsNotUsed" />
</nancyFx>
```