I have a very simple approach - if I don't meet the requirements for a challenge cache, I don't go out and don't sign that cache's logbook. In case I would be in a team of cachers that go out on a caching day and happen to look for a challenge cache where I personally don't qualify, the same, I would just not put my name in that logbook and come back once I qualify for it.
There are plenty other caches to log meanwhile and in this way I am not bothered by these existential questions about when and how to log what and how it would affect my statistics, making them more or less "true".
P.S. I know I'm not exactly answering the question because to me, both of the presented answers how to log these caches seem wrong in some way.
Example 1. You made a note 2 years ago and now, after filling the requirements, you are turning that WN on that old date on the into a find - that would mess up your find counter on all caches logged in that 2 year period and shift your milestones.
Example 2. You made a note 2 years ago and now, after filling the requirements, you are logging a new FI log with a current date - but your name is not in the logbook and you have not visited the cache since those two years ago. Maybe the cache is lost and archived, maybe it has changed location. Anyhow, you have not found a current version of the cache but you are logging a current FI log, also seems not quite right to me.
P.P.S. Some may say - "But what about those challenge cache locations that I may have never get a chance to visit again in my life?" - For me that doesn't look like a valid problem as well - there will be lots and lots of other chaches from the 3 million I will never visit in my life. Moreover, there will be lots of DNFd caches I will never have a chance to find later.