Updating table data from another table

Hello. I'm creating a database for a reporting system for my work and have a question I'm hoping someone can answer.

First of all, some details.

I'm using Access 2010 for the database, VBA for the coding and server 2k8 as the O/S.

Ok then, onto the problem. I have a collection of .csv files which contain the data I require. These .csv's are produced every day. What I'd like to do is to merge the contents of the latest csv, into a table which contains all previous csv's.
These csv's could contain the same data as the last csv as well as new data.

I've created an import routine which will import the latest file into a new table ready for merging, I just need some help with the actual merge.

As an example, I'll detail a sample of what each table contains.

(Previously imported table)
Key: IDNumber = 100
Open Date = 01/02/2003
Resolve Date
Close Date

(New table to import)
Key: IDNumber = 100
Open Date = 01/02/2003
Resolve Date = 02/03/2003
Close Date

As you can see, they both have the same unique ID, but the new table has some additional data I'd like to update the existing record with, as well as appending wholly new records. I don't want to iterate through both tables as the new data contains ~15000 rows, and the existing table has the same number of rows multiplied by the number of days data which has previously been imported! :s

I hope this makes sense and someone can assist!

Thanks in advance.