If I understand the question, are you asking whether 'get latest' will wipe all pre-existing data from your linked development database?
Rest assured that SQL Source Control will preserve your existing data in this circumstance. Tables will be ALTERed where possible. In some situations they will need to be rebuilt, in which case SQL Source Control will create a temporary table in which the data is copied, and then replaces it once the table has be recreated.
I'd encourage you to test the tool on a few of your own scenarios to fully satisfy yourself that the behavior is how you expect it to be, and of course let us know if it falls short.
Red Gate Software