A client has 2 separate desktop files. One with 2015-2019 data and one with 2020 data. Is there a way to merge these 2 files into 1 before converting to online? Otherwise one data set will override the other, right?
You are correct that two separate desktop files can't be merged into a single QBO online company.
You would have to merge them into one desktop company first. Without an accountant version fo desktop it is impossible to have both companies open at once and simply copy/paste. There are companies and maybe even self-apps that can do the merge (seek out Big Red Consulting)
Or you can leave the old data in desktop format as it can always be opened for as long as you have desktop 2019/2020 installed
If the data file has more than 350,000 targets, the file will need to be SuperCondensed first. It can then be converted to QBO. The SuperCondense service will shrink the size of your data file to almost 50-80% of the original file size which boosts performance, offers stability, speed and keeps you under the under the 14,500 name limit for Pro/Premier, helps you upgrade to new versions of QuickBooks easier and also enables you to convert to QuickBooks Online with ease. Use a professional third-party conversion provider for a solution or PM me for more information.