I don't know how much help I can be, but I've been there when DS has suddenly changed personality and caught you unawares, so wanted to let you know that you're not alone.
I think they hit an age when they start pushing boundaries in a really obvious and sometimes distressing way. He wants to see what you will do if he hits you, hits someone else etc... You and DH need to sit down and agree a strategy together, and then stick to it. We introduced the naughty step around about that time and I really wasn't sure at first, but we followed Jo Frost's advice on getting down to his level to give warnings, then if he needs to go on the step, explain why he is there (make sure nothing fun is around...!), then do a minute per year (we did one minute only for the first couple of months of 2yrs old - we wanted to give him chance to 'get it' rather than have a battle). The hardest part for us is not talking to him when he's doing his time - no matter how much he pushes you! Afterwards, make sure you get down to his level and check he understands what he did wrong and how you expect him to behave in future, then a kiss and cuddle to say sorry, and one for the hurt person too, if appropriate.
My DS is nearly 2.5 now and we very rarely have to use the step at all these days. It did help that I would allocate a step wherever we were, so he knew the same rules applied, even when out and about.
I sympathise greatly. I remember feeling like my gorgeous little baby boy had turned into a horrible little toe rag that no one wanted to be around. Just stick to your guns and deal with each situation as necessary. If, like me, you have a tendency to worry that others will judge you, I soon found that you are more likely to make it look worse if you appear to be doing nothing. Even what feels like a 'failed' discipline attempt (ie,he does it straight away again), has an impact because by repeating the process with him, he will realise you mean business.
It is hard work, but so worth it in the long run. If it's any help, DH and I have only been saying this weekend how lovely DS is and how responsive to our instructions - I'm sure I'll be faced with fresh challenges soon enough, but I'm enjoying my victories at the moment!
Good luck and sorry for blabbering on!