converting quicken windows to quicken mac... will brokerage account data cause problems?

@mikestone While we can answer what should happen, I don't know if anyone can tell you with certainty what will happen. In theory, your brokerage accounts will convert fine in Quicken Mac. Of course, you'll want to compare balances and holdings after converting to make sure everything looks correct.

The largest area of problems I'm aware of is with BuyX/SellX/ReinvestX transactions, which don't exist in Quicken Mac; these should import as two separate transactions, one for the action and one for the transfer, but some people have had a problem with them. If you have separate transfer transactions now, you shouldn't see a problem after the conversion. 

The best advice I can offer is to make sure your Windows file is in clean shape, follow the instructions for maintenance prior to conversion, and then compare the results carefully between the Mac and Windows versions once you import the data. (If you can run the two computers side-by-side, that can be a real help.) You may want to continue using Quicken Windows for a few weeks, in parallel with Quicken Mac, as you learn your way around the Mac program. You can always re-convert your Windows data file another time if you find things which would be easier to clean up in Windows than post-conversion. 

No, you're definitely not doomed to failure if you can't do maintenance on the Windows data file; it's a just recommended step as it may prevent minor corruption in the Windows database from causing problems during the conversion. 

The two versions of Quicken are completely independent with regard to your banks. (In fact, you could have two data files on one of your computers, and they would each be independent of each other.) Both versions of Quicken will download whatever data the financial institution has available for download. Thank bank doesn't keep track of what you have downloaded previously; Quicken does the heavy lifting of trying to prevent duplicate transactions from multiple downloads. You might find some duplication in your Mac data file shortly after conversion because your connection method from the Mac will be different than the connection method from Windows. (Each financial institution assigns a unique ID number to every transaction, but across platforms and connection methods, it's possible for them to be different, resulting in some duplication in the initial download from each bank.)