If a HT wants to make a maintained school an academy, he/she must go to Governing Body. It is the GB that makes the decision following a consultation with stakeholders. Sometimes, the DfE force conversion. At that point the GB can make the decision which chain to go with as this is required in a forced conversion.
Terms and conditions for new staff into an academy are dependent on what the academy has decided to do. There are a lot of academies that have chosen to stay with the say pay and conditions that maintained schools follow. However, be aware that the government is messing with those conditions every time we turn around!
An academy can change the length if the school day and length of the school year. But, for staff, contracts usually also have to be changed and financially compensated to reflect this change.
As far as getting rid of staff. On the occasion of a failing school converting, I have seen poor staff removed. However, in the chain I work in, I have only ever seen staff removed by the same procedures that are followed within maintained schools for poor teachers.
Not all academies are created equal as no academy chains are created equal. There are some amazing academies and chains out there that are doing what is right for all.
What has had massive negative effects on schools and academies alike is the seemingly weekly change in how we do things handed down by the government that is making life as an educator miserable.