One of the Stock Complaints about NSF review is how people feel as though the goal posts are always moving because the panel composition changes every round. While that's a topic for a different day, the preproposal system provides junior people with a bit of an exception here.
Preproposal results should be filtering in in late April or so. The CAREER deadline is in late July. It might not be the four months that PIs are given to turn an accepted preproposal into a full proposal, but that still leaves about three months between hearing back about your uninvited preproposal and the CAREER deadline. Why should you consider packaging your proposal for the CAREER award?
For one, the CAREER grants get mixed in with the full proposals submitted to a panel, so it is essentially a backdoor access for junior people to get their full proposal looked at. Yes, the CAREER has certain stipulations that are going to require your attention, but it's a good idea to be in the pool of proposals that are being considered for funding.
Second, and maybe more importantly, you have feedback from many of the people who will evaluate it again! IME, there is decent panel consistency (maybe ~50%) between the preproposal and full proposal round. In cases where someone can't make the second panel, they are often asked to be an ad hoc on a few proposals they saw as panelists. It is to your significant advantage to robustly demonstrate that you have dealt with the critiques of the reviewers rereading your proposal and that it is better now than ever!
Now, you're obviously going to want to be choosy here on which proposals to try this with. You only get three shots with the CAREER proposals, so you will need one that you feel confident just missed the cut and can be turned around effectively. Sometimes that can be a little hard to judge, but a careful reading of the reviews and maybe a discussion with the PO might help you make a call.
But don't ignore the opportunity.