Josh, this sounds like the opposite of what Method is designed to do. Generally speaking, Method is only looking at "current" QuickBooks data, and it wasn't intended to have years of transaction history in it. Also keep in mind that not all data ia allowed to be synced from QuickBooks to Method, so keeping the transactions in Method might result in incomplete records. You might also skew your customer balances in Method if you only kept invoices and not payments.
Why do you want to archive the existing data? The file size is not outrageous (I would not even call 150 MB very large if you are on a current version of QuickBooks) and you aren't close to the limits on the number of customers. Are you experiencing any issues with your QuickBooks file? If so, look at other remedies before you archive. There are many ways to speed up the file if that is the only issue. Talk to a good QuickBooks consultant about that. Start by looking at your computer and network specs to see if there is a bottleneck there.
If you do decide to archive past transactions, why not just make a copy of your QuickBooks file before you do it so that you'll have the details available if you need to look them up?
-Mary Longacre
703-299-0802