This was the most painful part of moving from Quickbooks desktop to Quickbooks online. My best advice is to do this after closing the month and reconciling everything. Make backups. Make sure everything is reconciled and your bank accounts and QB match exactly. Be prepared to spend the good part of a day checking this and getting it right as it will save you a ton of time fixing things later. Call for help if you need it, before you make a mistake.
I really like QB Online, but the move from the desktop, for this very reason, was very painful and cost me over $1000 in accounting fees to get it fixed. This is QBO's weak point, and it's a strong weak point. Take the time and get it right.
Sync all accounts.
reconcile all your accounts
Then transfer data to QBO
Check this to see if the functions you rely on in QBDT are even in QBO, a non partisan comparison of QBO vs QB desktop
Along with what belle said, when you do get QBO up, compare the QBO P&L, balance sheet, inventory valuation summary (if you have inventory), and sales tax liability as a minimum, compare the QBO reports to the same reports in desktop
Some things do not transfer from QBDT to QBO read the limitations
IF, if you are using inventory type items, before you do the export to QBO, in QBO be sure to turn on qty tracking in company settings - if you do not inventory items will be created as non inventory.
sales>products & services
and turn on qty tracking (inventory)
in company settings>expenses>bills & expenses turn on the items table and purchase orders.
Then when you enter an expense or bill, about half way down the left side is a title "item Details" click that and a table opens where you can select the item, qty and item total cost for purchasing inventory
and see the pic for a new inventory item set up, if you need it after the import
And if you are going to import desktop inventory to QBO, see this for how to fix the import before you start entering transactions - I have not done this in a while hopefully it has been fixed - the symptom is that in the products & services list each inventory item has sub-items titled for sub accounts.
Although conversion is from QuickBooks to QuickBooks, these two are very different software programs, needs to recognize that part first. They've improved a lot over the years, the "Export company file to QBO" process but still needs to know about export limitations and what to do after import into QBO.
Yes, this. It's important to understand before you convert your QB desktop to QB Online that not only are these two completely different software programs, but they are like two completely separate companies. They don't talk to each other. Don't go into a conversion thinking it's the same SW program and the same company. There is a wall between them. If you have direct deposit banking, it needs to be setup again, separately and create a new merchant services account for the new Quickbooks Online. If you have QB payroll, you need to re-enroll with QBO. They may as well be two different companies. Nothing transfers over. It's like changing banks. You close one set of accounts and open a new set of accounts. It's a major change, not just a simple switch over.