Tier 2 General, changing sponsor, which form to use

[manibest]

I am Tier 2 general and now changing my sponsor.

Q1) Which application form do I use to change my sponsor:
1) "Extend your application in Tier 2" or
2) "Switch to Tier 2 from another Visa"

Q2) Do I need to provide funds maintenance and English language proof again?

Q3) Which is the preferred way to apply: a) By post or b) Premier service at desk?

Thanks