After getting this question, here is what I asked the BLM via e-mail:

11/24/2003 10:36PM
Subject: Credit from Weekly Passes
Neil,

This question has come up: If a person buys a weekly pass, can that $25 be
credited toward an annual pass if they bring in the receipt? I would think
it would be OK if it was one week only being credited. Would it need to be
done at the Ranger Station?

Thanks,
Glenn

Answer from Jamie Neilans at BLM:

Tue, 25 Nov 2003 09:02AM

Yes, they can credit the $25.00 week pass towards a season pass, as long as
it is within 10 days of purchase. They need to send it to Central Parking,
PO Box 671, Brawley, CA 92227 with their receipt, address, and contact #.
Once Central Parking receives the information they will call the contact #,
get the person's credit card # in order to charge the difference, and then
send them a season pass in the mail.


So, there you go. Good Deal! Get it to Central Parking within 10 days of purchase.