Updating original files following post de-dupe updates

HenryG95 You might consider using a secondary helper table which translates the old address to the new.  The complicating factors in this approach are 1) what to do when either a new customer moves into the old address and 2) how to account for only one member of the family moving to a new address.  Ignoring the second complication, this could provide one of many possible solutions:

1) create a family group column in your original data based on last name 

=MID(A2,SEARCH(" ",A2)+1,65)

2) Instead of entering the new address in the original table, enter it into the helper table

3) Use a formula looking for a match on both last name and original address to bring over the new address.  This formula would be entered in the New Street column in your original table (of course, replacing the static text with the table references in the original.  Doing the same for New City & New Postcode.  OR, you could leave the new address columns in place and create three new columns for New Family Street, New Family City & New Family Postcode, particularly if you want to use power query to maintain your helper table (step 4)

=XLOOKUP("Andrews25 Privet DriveLondonB1 2XR",Table1[Last Name]&Table1[Old Address(full)],Table1[New Street])

Helper table & lookup formula

 4) of course, since you have tens of thousands of records, you could use power query (Get Data/From Range) to pull in the original table, group by last name, Original Combined Address, New Street, New City, New Postcode, then filter out blanks in the new address fields.  This should give you a starting point (A new helper table on a separate tab) of unique last name & old address combinations which also have a new address. 

Happy to elaborate more if you're not familiar with power query.

Hope this helps get you started.