I'd check with your USDA vet. I'm taking my paperwork in next week to get it endorsed by them, and they want official documentation (which I'm scrambling to find for my dog who got chipped 3 years ago) listing the date and chip number.
You may be able to get documentation from HomeAgain, perhaps, with when the chip was put in, though it might need to be signed by a vet stating that the chip was inserted on that date.
Otherwise, if your USDA vet is lenient, you might be able to get away with a letter from your vet stating that s/he scanned your pets on X date (12/31/09, or whatever) and their chip #s were XXX and YYY, and then show that you had the rabies vaccinations/bloodwork done on Y date (after 12/31/09, in this example).
Also make sure that the chips are still there...my cat's chip couldn't be found when we scanned him, though he got chipped a few years ago. I had to re-chip him and then do the vaccinations.