@DougHiggs I hear you. And agree with @BrianKelly about doing a small test first. My experience, depending on the size of the client and their transactions, is that it could take hours. The last client I did something like this was a bear. We merged 100 items at a time, and it took over 5-hours for each grouping. Needless to say, it took coordination with their billing department to update all invoices before they left each evening, so the utility could be run. If I recall, I billed them 90-minutes per evening at 1 1/2 time our normal billing rate, for 18 evenings. When it was all done, between paying their billing clerk overtime to stay at night and update the batch, and paying our services, I think they would have been ""OK"" with the ""noise"" of the unused items. They never asked me to do it again.
Oh, don't forget to check on their nightly backup. Doing this after the backup would be better. Just incase, and it should be faster too.