questions reg tier2 visa extension and change of employer

[kannaiah]

Hi,

I got my Tier-2 Gen work permit in 2010 and started working from mid of June, 2010.
I have following questions (I'm not well aware of the jargon used because that application was entirely taken care by my employer)

1. Can I change my employment?
2. Assuming the answer for the first question Yes, Would I be subjected to new rules or old rules for my PR (I think that's after 5 years of stay)?
3. How many years my current tier2 Gen would get extended? 2/3yrs?
4. What happens if I cannot complete 5years even after extension? (as I started 1 month late initially, and took holidays)

Please help with the above questions.

Thank you.

Regards.