Also, just a tip based on my experience, consider how much time you have available when you sit down to start updating and if you have a fair chunk of time look to get classes with hundreds of locos out of the way - I broke the 66 class down into their sub classes which helped but it still took me ages to complete them. And as for the 08 shunters - give yourself a good few hours and be prepared to see '08' appear every time you close your eyes afterwards
On the other hand, if you've only got a short amount of time then just focus on getting the smaller fleets done - 88s for example will only take you 15 minutes but its another class available to everyone