Named versions, and a diff you can actually read
Up to 50 named CV versions, one per airline, with a readable diff against your base CV
Save the CV you are editing as a named version, one per operator, and load any of them back. The library sits beside the working copy rather than replacing it, so the single CV the free builder holds is never touched by any of this: a failed save cannot damage the document you are actually editing, and if access ever lapses the library is hidden rather than destroyed. It holds up to 50 versions per account, and the save after the last slot asks you to delete one first rather than silently dropping the oldest.
The reason to keep them apart is the question you have three weeks later. Six tailored CVs look identical at a glance, and what you need to know is what you changed for this one. A raw comparison of the saved data answers that in a language nobody reads, so the diff is written in the form's own words: which field, what it said, what it says now, with both values shown. Summary changed is useless. Summary: this sentence became that sentence is the answer.
Two rules keep it honest. It never says which version is better and never suggests one, because that is your judgement and not a tool's. And a blank is treated as a real value: clearing a phone number is reported as removed rather than skipped, because an emptied contact field is exactly the thing worth catching before an application goes out.