Assumptions:
your spouse goes on Medicare on July 1 (I don't know if it is this date or August 1 - go check).
You go on a Self-only HDHP policy on July 1.
You two have a Family HDHP policy for 6 months - this means (6 x $8,550 + $1,000) / 12 or $4,775. This amount is shared by both spouses in any way agreeable to both.
You will have a Self-only plan for 6 months. Normally this would mean (6 x $4,300) / 12 or $2,150, EXCEPT that you will have HDHP coverage on December 1, so this means that your annual HSA contribution limit is $5,300 ($4,300 plus $1,000 additional contribution) or the remainder of the Family coverage (zero) plus max Self-only $4,300 plus $1,000.
As I said, you can adjust the Family coverage any way you like.
You should each have HSAs. If one of you does not, get one now. The $1,000 additional contribution can go only into the HSA of the owners who are 55+. The $1,000 for your spouse is prorated because your spouse stops being eligible when your spouse goes on Medicare.
So it appears to me that you two can contribute a total of $10,075 between the two of you. NOTE: you can't put all $10,075 in one HSA - that's why you need two. At the least, the $1,000 bump for you being 55 or older has to go your HSA, and the prorated $500 bump must go into your spouse's HSA.
Of course, if you change how the Family coverage is allocated, this may change some numbers.
This calculation comes from the instructions for form 8889.
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