Question about Medicare Supplemental and "Open Enrollment" [View all]

I have AARP/UHC supplemental Plan F. Fantastic plan, I haven't paid for any medical expense for over 10 years.
The premium is $350/month before the next increase, which is coming soon.
I just came back from visiting with a SHIP counselor-- (person trained in helping with medicare and insurance.)
I wanted to look into changing plans to have a lower premium [this premium is killing me! I am a very low-income person], and co-pays.. I am healthy at age 78 and do not have a lot of doctor visits, so I wanted to see which would be better.
The SHIP counselor told me that I can't switch plans until Open Enrollment in October!
I am devastated. I was already thinking about what I would do with the extra income a lower premium would give me.

My question-- Is this correct? Because I also have been talking to a UHC agent who tells me I can switch today and have a new policy in a week.
Both cannot be right.
Obviously I have to call the insurance company to get a definitive answer, or look it up -- but these two prospects I find very daunting, so I thought I would ask here first, and go from there.

Thank you for any info...