The only update carried out by Chimpegration is what is shown in the review screen. When two fields match in value, they do not appear in the review screen because they are the same, so nothing needs to be updated. For example if my first and last name in MailChimp is "Dave Zeidman" and in Raiser's Edge it is "David Zeidman" then in the review screen you will see one line asking you which direction you want to change the first name. Should it be "Dave" or "David" in both places (in the case of import you would decide whether or not to change RE or to ignore the value)?
Addresses are no different. If my city and postal code match but address lines do not then only they will appear in the review screen and only they will be updated.
However there is a problem here. We offer the facility to back up the address. This was done on the understanding that the incoming address would be different and that MailChimp would be adding an entirely new address. In RE the address is backed up i.e. it is set to an alternate address and is no longer the preferred. A new blank address is added in its place. The problem here is that if only the address lines match then the only update done by Chimpegration is to add the address lines. All the other address fields remain blank. You are left with an address that is only partially populated.
Following feedback from our users we have decided to change the way this works. We will back up the address but retain the original address field values in place. If there are fields to be overwritten by Chimpegration as per the instructions in the review screen then they will be updated. This means that even if, for example, the city or postal code is not in the review screen, they will not be blank but will have the same value as previously. We believe this is the most likely scenario.
There are, however pitfalls with this too. What happens if you do not map one of these fields? Well there is a risk that you will retain erroneous data i.e. if you do not map city but the subscriber has clearly moved away, then your city will still be the original value.
Having discussed this in depth, we feel that this scenario is relatively uncommon and much less likely that the former scenario. When working with addresses it is always advisable to map all the address fields.