Content update & translation
From a technical perspective, you definitely can update your source files while translation is still in progress. Transifex is designed exactly to facilitate agile development and help translate faster. There are multiple ways to set up automation for updating content, as described here (manual, automatic, through API and CLI, GitHub integration). Transifex recognizes the difference and your source file is always up-to-date in Transifex.
Our recommendation is to make sure that your translators are aware of this “continuous localization” mode and possible multiple changes in the files so that they don't get surprised that some strings got added or some were deleted. Kindly inform your community/translators or talk to your LSP about this mode of updating the source file, so that they can adjust accordingly.
A few tips for the translators:
They can enable a "watch" feature and get notified when changes in the source file happen;
All previous translations are stored in TM and will show in the “Suggestion” tab, if the match with the updated string is high;
All translation activity is reflected in the Translation Reports: even if the string that was translated was deleted or modified, the translation activity was captured in the report.