I understand the efficiency of being able to clean up mistakes in QBDT, Victoriah. 

In response to your first question, yes. The condensed data will still be available and will show on reports as needed.

 I'd like to add what Paul mentioned about the Client Data Review (CDR) tool. The CDR tool is one of the best practices for clean-up tools as it enables users to fix common errors in their data.It offers plenty of benefits such as removing old, closed transactions, and helps reduce the file size without affecting the integrity of your financial data. 

It's important to note that access to the Client Data Review (CDR) tool is available in the QuickBooks Desktop for Accountant version. Therefore, I recommend reaching out to an accountant who has this version to utilize this tool. 

Regarding your second question, if you encounter an issue with insufficient room when attempting to update your QuickBooks Desktop, you can either condense your data or create a company file if condensing your data isn't possible.

For further details about CDR, you can visit this link since it has some steps and education on what you can review in QBDT Accountant and Enterprise: Accountant tools: Review list changes

For further questions aside from Client Data Review, please feel free to ask the Community or comment. We'll be here to assist you.