It depends on your policy. The policy will have either guaranteed or reviewable premiums. Whilst the guaranteed premiums stay the same for the life of the policy, reviewable premiums can be changed every, for example, 5 years.
Also, there doesn't necessarily need to be a change in personal circumstances for premiums to be reviewed upwards - it could be down to the performance of the insurance companies investments.
"In case one of them dies before the other", in my opinion, isn't sufficient. You need to see a need and get insurance to fulfill that need.
As an example of what I mean, your parents are likely in receipt of a pension. You need to evaluate their expenses, determine how much of a difference there is between their pension and the required income should one of you die.
Then multiply that shortage by the number of years life expectancy of you both (possibly adding a little for comfort). The number of years life expectancy together with the result of this calculation should be used to select a value and term for your life insurance.
Too many people pick a random, even number out of their heads and a random term - for example, 100,000 for 20 years is a popular policy (although it's impossible that everyone with such a policy has the same needs - or even close).
If their pension is sufficient, in particular if one of them died, then no insurance is needed and you can avoid the premiums which, for a policy with as low a value as your parents, will be covering mostly administration of the policy as opposed to the sum insured.