I believe I know the challenge of which he speaks. In fact, if I have the right one in mind, I've completed it. Basically, an alphabet challenge. But the "placed prior to" requirement in question was to keep people from going out and hiding caches for the seldom-used letters.
It made it harder and I gave the CO some good-natured grief about it. That said, I am not one of those people that thinks "it was hard for me, it should be hard for others." My vote is to drop the date requirement all-together. If I had my way, when they came out with the current list of guidelines for challenge caches, all older challenges would have had a year to either comply or make a special case for being allowed to be grandfathered. And I would have wanted the reviewers to be TOUGH and have that be a rare case.
If you wish to keep a date requirement, then by all means, check with the reviewer about advancing the date some if you can.
If you do move it up, maybe instead of making it an anniversary date of when you hid it, go a little further than you meant to and choose something like "Placed before Jan. 1, 20xx." The date of when you hid it made sense at that time. But an easily remembered date would be so much better for current searchers. There are requirements that "up the difficulty level," and there are requirements that are just "annoying." Working with a date that is random or the anniversary of the cache being posted is just annoying, not really more difficult. It adds nothing to a sense of accomplishment that it had a date that to the searcher feels random vs. really dealing with a year. I remember every time I planned a trip out of town I took a few moments to check my PQ's for qualifying caches for letters I needed. Because that date meant nothing to me, I had to check on that date every time so I could filter my list and look. I could have remembered a YEAR. Of course, if I had skills for writing GSAK macros, maybe that would be different. But I think the list of macro writers is much MUCH shorter than the non-macro-writers.
But I'd check with the reviewers to see if any changes would alter the grandfathering.