I have a client who has an online account where he generates invoices.  His previous bookkeeper did not give me that file.  I constructed a new company file on my QBDT for Mac for his 2023 year.  The online file had prior year's, mid-2021 and all of 2022 and a few months of 2023 on it.

I would like to upload my file to his QB online file for that company.  How will that work?

Will there be duplicates for the 2023 entries?  Will my file, upon upload, erase the prior entries already in the file or will it append to what's there?  If I have a customer list with different information, for instance, addresses where she had none or vice versa, how will that be treated upon upload?

Many thanks for addressing this perplexing difficulty.