Importing new customer table into existing Quickbooks application

I'm fairly new to Quickbooks, but pretty experienced with Excel and
relational databases. I'm helping clean up/scrub customer data and,
because there's so much clean up to do, I figured if I did all my
editing in Excel, I could import the Excel data and overwrite the
existing Quickbooks customer data.
This Quickbooks application is currently in use for 2009 business. I
don't have a test environment to play with. If I do this excel data
scrubbing and I Import into Quickbooks, will I kill/screw up/ destroy
the current year's open invoices, estimates or books & records in any
way? The owner of the business I'm helping doesn't have a clue.
Thanks to anyone who can help.

What exactly are you importing--Customer lists, invoices, payments? What method are you using to import this information--QB utility, iif file or a third-party tool?
Importing customers lists will only ADD customers if they don't already exist. For existing customers it will over write the info contained in the excel file. It really can't be used for clean up purposes.
As for "test" environment, put QB on another computer or make a backup of the existing file and change the file/company name and use that file as your test environment.

How did you determine that it would be faster or easier to "scrub" Quickbooks data outside of Quickbooks? It sounds more like all you have in your toolbox is a hammer, therefore you approach every problem as if it were a nail.
Create one.
If I do this excel data Possibly; it all depends on what you do and how you do it.
Try the following Intuit support web site articles: KB ID# 1005472 and KB ID# 1003348
-Mark Bole

The existing customer data in QB is a mess....wrong addresses, no phone numbers, duplicate customers, some Bill To addresses are last name first and others are first name first. I'm trying to figure out the easiest way to do a massive clean up without going through 1 by 1 in the customer Center to fix the data
Oh, I'll use the QB Import Function and I'll either just import an excel spreadsheet or I'll save it as a csv file, whichever works best.
If I have to go through and inactivate all the bad data/customers 1 by 1, I'll do it.
Data integrity is SOOOOO key!

thanks, Mark.

Okay. This should not be too difficult but I am not sure what the value added you get from doing this data clean up especially for non active customers. For current customers I ususally just clean them up when I do an invoice. I'm not one to create billable hours just because someone feels the customer database is a mess but that's just me.
Run the Customer list report but modify it to include the columns that need revisions (to keep things simple). Dump it to excel. Get rid of any records in the format Customer:Job as they can not be imported-only customers can be imported. Make your changes in excel. Get rid of any records that you do not need to revise. For addresses, make sure you include the pieces of street, city, state, zip as individual fields rather than 1 total field. Bill to fields are stored as one string but address is not.
Then select File>>Utilities>>Import>>Excel Files and select the Advanced Import feature. This allows you to select which fields you wish to import and their order.
And of course, make a backup of your file before you do any importing. That way if something goes wrong you can start over again. I would do a handful of records at a time to keep things managable.
The existing customer data in QB is a mess....wrong addresses, no phone numbers, duplicate customers, some Bill To addresses are last name first and others are first name first. I'm trying to figure out the easiest way to do a massive clean up without going through 1 by 1 in the customer Center to fix the data
Oh, I'll use the QB Import Function and I'll either just import an excel spreadsheet or I'll save it as a csv file, whichever works best.
If I have to go through and inactivate all the bad data/customers 1 by 1, I'll do it.
Data integrity is SOOOOO key!