とあれば is used to presume a certain context for the following clause or statement. Whereas としたら、しすれば、とすると can be used for actions that the speaker would take, the clause preceding とあれば usually involves an action by another person.
Grammatically, it is very close to ならば.
私は引っ越すとしたら東銀座に引っ越すだろう。
If I were to move house I would move to Higashi-Ginza.
私は引っ越すとあれば東銀座に引っ越すだろう。(✘)
If I were to move house I would move to Higashi-Ginza.
お望みとあれば何でもします。
If it is your wish then I will do anything.
必要とあれば弁護士に契約を修正してもらう。
If it is necessary, I will have the lawyers amend the agreement.
とあれば is used in both written and spoken Japanese and has slightly refined ring to it.