There is a medicare "guaranteed right" regarding private insurance medigap plans that appears to apply to people who have been continuously buying IBM secondary insurance. That means the medigap private insurance provider does not have a right to deny you coverage for preexisting conditions if you want to move to a medigap plan. Even though IBM secondary insurance is not government sanctioned medigap insurance the fact that it is employer supplied insurance puts it into the guaranteed right plan coverage. Since IBM offers the plan every year with "slight changes" the fact that it changes is what enables the "guaranteed right" capability to be activated.
OK - what does that mean. IBM's secondary insurance is really disaster insurance. The premiums are low because they are betting you will never meet the thresholds to make a claim. You have to be pretty sick and pay a huge out of pocket expense before it kicks in. If you face that situation then enroll in a medigap plan during IBM's open enrollment period and switch to a medigap plan for the following year. The medigap premiums are substantially higher but the medigap insurance kicks in right away to pay copays and deductibles. In that sense having IBM's secondary insurance until you need better is a way to be sure you have access to the best coverage available should you need it.