“How much would it take?” – Yeah, it is interesting question, which we gave to ourselves being maybe 3rd year of developing projects based on NHibernate. We took quick look and from top of the head it looks like it would take a lot of time, because FluentNhibernate has changed syntaxes, because there might be something not working after changes, etc.
You know what? I was able to change version of NHibernate and make it working for us within 25 minutes.
It just proves known statement about giving estimates: “Judge as last resort!” Better compute and much better count if applicable.
So what is needed to be changed when you are upgrading to higher version of NHibernate w/ FluentNHiberante (from 2.0.0.4000 to 3.1.0.4000):
  1. Download compatible versions of NH and FNH. Usually FNH is step behind. (see release notes)
  2. Update references of your projects if you have folders for versions of 3rdParties.
  3. Update fluent initialization of NH with adding proxyfactory, like below:

var configuration = new Configuration();
var properties = new Dictionary<string, string>();
properties.Add(“proxyfactory.factory_class”, “NHibernate.ByteCode.Castle.ProxyFactoryFactory, NHibernate.ByteCode.Castle”);

4. Also you would need to reference NHibernate.ByteCode.Castle in your starting project.

5. Update fluent mapping with removing different noise code like below (if you are below 1.0 version):

image

We had about 250 mapping files so “Ctrl+H” had to be used a lot.
6. Run your application and see if you don’t have any issues.

So, long story short, updating to higher version of NH is not painful. It is still not clear if it has big impact or not, especially for big project.
If we decide to upgrade I will let you know if testing shows big impact.