Turning 65: Here Are the 7 Critical Things You Need to Do Now

Be aware that there is a late enrollment penalty for Part B Medicare if you are age 65, no longer covered by your own health plan but are covered by your spouse's health plan. You can avoid this late penalty by providing evidence to Medicare of your coverage under your spouse's health plan. There is a form you will need to furnish to your spouse's employer to have them certify your dates of coverage under their health plan. When your spouse is no longer covered by an employer health plan, you will then be able to enroll in Part B without a late enrollment penalty.

RE: Medicare and Medicare Advantage, moving back and forth. It is complicated. Some states including CT allow movement back and forth from MA plans to Medigap plans without underwriting or medical rating or age rating.
The other states have all sorts of rules that protect insurance companies from adverse selection. When turning 65, the Initial enrollment period allows entry to Medigap plans without underwriting in all states, but moving from plan to plan after that can be fraught with underwriting problems and increased premiums because of age.
Special enrollment periods can occur which allow movement and Guarantee Issue pricing but still restrictive in some states. Ma plans do not have rating or underwriting mechanisms and readily accept members during special enrollment periods,

Special enrollment periods Include, leaving employer plans, moving to new service areas, or a variety of losing coverage situations.

Tom Cretella, CLU at Cretella and Belowsky. https://www.cretellabelowsky.com/

A guide to switching Medicare plans: https://www.senior65.com/medicare/article/medicare-insurance-switching-ultimate-guide#mss

I'm turning 65 in April. I've found Medicare to be rather overwhelming to make a decision on. I canceled getting my employer insurance, because I don't like the high deductibles when I'll never spend that much $$ in a year. The most I spend is on my chiropractor. I use my FSA card to pay him. He advised me not to consider the Advantage because it doesn't pay him. Can't afford to do that! I need to make sure I choose the right insurance for Part C and D. I wasn't aware there is a prescription plan that's separate & apparently doesn't cover my current prescription - Restatis.

Yolande, you may want to contact your local State Health Insurance Assistance Program
https://www.shiphelp.org/ They can help you choose the plan that fits your needs. There is no cost for their service and they are not connected to the healthcare insurance industry. If you go with a Part C/ Advantage policy most have drug coverage as part of their plan so you may not need Part D. Lots of discussion about Medicare plans on this site.