Scheduled a colonoscopy but changing insurance - please advise :(

I need some advice...

I'm going on my employee health insurance in January. According to the woman at work who handles it, it will go into effect January 3rd. I'm currently paying out of pocket for health insurance, and I have a colonoscopy scheduled for January 5th. I don't want to cancel my current insurance until I know I have coverage from my employee one. The problem is, the employee one is an HMO that requires a referral, while my current insurance does not. I scheduled this appointment without a referral.

So, I really don't know what to do. "HR" woman and my head "boss", both of whom are unaccommodating and in general not nice people, told me that I will be denied health insurance because I did not put an end date on my application (since I didn't know when the new one would go into effect) and/or I checked off the box saying I won't cancel it (I honestly don't remember...). Anyway, is this true? I thought you can have a primary health insurance and a secondary? I honestly don't trust the words of my boss.

And this leads to another problem... should I pay out of pocket for the month of January and cancel in February? Risk having two insurances in one month? Reschedule my appointment until I get that health insurance card? I'm on Prednisone right now because I had a flare up and will not have had an exam in two years, so would it be a problem if I pushed it back?

Ahhhhh, I'm so conflicted and stressed over this, and this flare up couldn't have come at a worse time! I've been trying to schedule an exam with the GI for December, but the only dates he has available are I'm out of town for the holidays.

Thanks for the advice, Dr-A. I really appreciate it. Should I call both insurance companies? I usually like to avoid having to deal with them... especially on the phone. Ah, the necessary evils...