You are confusing two different things.
You are allowed to spend HSA funds for qualified expenses, for yourself and your spouse, regardless of the kind of coverage you have.
Overfunded means you contributed too much. HSA accounts are owned by individuals. If you went on Medicare in April, then you can only contribute 3/12 the limit to your account. But if your spouse is also covered by an HDHP and has no other disqualifying coverage, then she could contribute to a separate HSA in her own name up to her contribution limit.
