Update Contact Address via Field trigger

Summary:

We have a requirement to update the Contact Address based on account associated with the Contact.

Content (required):

We have a groovy to do this. There is no issue when we select account for first time. The address of account is stamped on contact. But, when we change the account, address is not getting updated. Same script works in Before Update trigger but not in Field trigger.

Also what is the best way to remove address. If we remove account from Contact, then the contact address should be null.

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Code Snippet (add any code snippets that support your topic, if applicable):