The biggest transition I've gone through recently is going from not being a parent, to becoming a dad. Pretty big transition. Kaleb is only 9 months and he has been through many more bigger transitions than my one.
Aside from the obvious huge one of living inside Liz to becoming his own entity, he has had a number of major changes in his life - drinking only milk to also eating solid food, not being able to move to learning to roll over and then crawl along the floor are quite massive changes to go through. Having no teeth to having some pop out your gums is probably the most painful transition I've seen him go through.
Learning all these new things aren't all major changes, some are small on their own, but together indicate a big change. Earlier today Kaleb was having fun crawling round his room. He's still doing his "commando" style crawling instead of the conventional style and for a while has been able to go from sitting to laying on his front to crawl. Today, however he went from crawling to pushing himself up so he was sitting again. Before he could only transition the one way, but now it turns out he can do both. A fairly small change, but part of the bigger, and still ongoing transition of starting still to moving about on his own.
One thing I am glad about is that I don't see being a dad transitioning away from being interesting any time soon