This sort of term insurance could be viewed as a deferred selling method employed by insurance companies in the hope that you will buy investment-type life insurance at a later date.
The way it works is that on specified dates (agreed at the outset), you have the option to convert your protection only policy into an investment-type insurance policy based on your health at the time you took out the original term insurance.
This option is of limited use and it ties you to the products of one insurer from which you may not get the best deal.

However, if you are older and/or your health isn’t what it was - and you are considering switching from a term insurance policy to flexible whole-of-life policy – there is no harm in comparing the price of what your current insurer offers with what is on offer elsewhere but bear in mind that premiums may have to go up in the future.

See also - Increasing Term Life Insurance and Decreasing Term Life Insurance