Yes. You can and there are no restrictions to change the module from Accounts to Accounts & its Contacts and vice versa.
However, if you want to change the sync type from Contacts Only to Account / Account and its Contacts and vice versa, you will need our assitance. We have to make changes in our end to change the sync type for you.
For instance, if you wish to change the sync type (Between Zoho Books - Zoho CRM) from Account and its Contacts to Contacts only, we can enable you to set up the sync in two ways depending upon your preference:
Write back to us at support[at]zohobooks[dot]com mentioning your preference and we will do the necessary changes in our end to enable you to set up the sync.
and GST filing.