What Tom W said. However in answer to your last question: Replace Data Source can (and often does) reset all the workbook formatting to Tableau's defaults. So if you've done lots of formatting, then you need to use this trick to hopefully avoid this issue. This trick is brought to you by Matt Lutton. Before replacing, go to (click on) each tab (worksheets & dashboards) in the workbook. Then the replace should 'keep' your formatting (but no guarantees).
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Another thing about REPLACE DATA SOURCE. You might lose calc fields.
Calcs that are on existing sheets will get carried over to the new data source, but those not on existing sheets will get dropped. This includes calcs that are used in other calcs. The final calc might be on a sheet, but some intermediate calcs may not. You might lose them (which would break the calcs that depend on them.)
To avoid losing calcs, create a new sheet. Drag all your calcs onto the data shelf of the new sheet. Doesn't matter what the final viz of the sheet looks like. Just get those calcs onto a sheet.
Then do your REPLACE.
Afterward you can just delete that placeholder sheet.