That link didn't describe my exact situation, so it listed many additional steps that were not applicable. My banking and "property & debt" accounts and transactions were already imported to the new data file with the QXF export/import. Here is what I did to get my investment accounts and transactions after extracting the relevant information from the linked post...
@Todd Nicholson said:
"That link didn't describe my exact situation, so it listed many additional steps that were not applicable."
Since it would be literally impossible to list every Quicken user's exact situation; the link was intended to provide the appropriate "considerations" that might be applicable to anyone attempting to "merge" Quicken files ... intending to allow every user who read it to choose the considerations that applied.
Then @ Todd Nicholson said:
"It was the warning referenced in step 12 that made me think that this wouldn't work."
The warning you refer to, which states "QIF import is not available for checking, savings, credit cards, 401(k) and all other brokerage"; is a misleading warning - it is not literally true, but it must be taken into consideration by users of Quicken versions from about Q2004 through Q2017.
If you were able to import an investment account QIF file into Quicken that had not be modified following its export from Quicken, then you must be using Q2018 or later.
Otherwise you would have been required to take action to modify the QIF file to allow it to be imported to a Quicken investment account by following the instructions in this FAQ:
https://community.quicken.com/discussion/7150750/faq-how-to-import-qif-files-into-non-cash-accounts-post-q2004
That FAQ is applicable to all Quicken versions from about Q2004 through Q2017 (those who don't see the warning when they initiate a QIF file import, should not have to consider it).