Yes. You are free to change the name of a limited company. However name search has to be carried out to ensure the new name is still available.
Once the name change is approved, a "Certificate of Incorporation on Change of Name " will be issued by the government.
Contracts with old company name need not be signed again. However, new name need to be used for all new transactions and therefore new set of company chops need to be made.
Authorized share capital
Yes. You can increase (or even decrease) the authorized share capital.
The standard authorized share capital is HK$ 10,000 splitting into 10,000 shares. For every HK$ 1,000 authorized share capital, you need to pay HK$ 1 duty fee.
This means you have to pay HK$ 1,000 duty fee to increase authorized share capital of HK$ 1 million. There is no limit on the maximum amount of authorized share capital. After the increase, you can freely allot them to shareholders.