You can upgrade or downgrade the tier of a sensor at any time. However, it will only take effect after the next eligibility check. Just be mindful of the fact that the bonus rewards are paid exactly 365 days after the day when they were earned, one day at a time.
This means that, for example, if you join the HODLER program on February 1st, 2023, with a sensor in the Diamond tier and then you change it to the Gold tier on March 1st, 2023, you have to make sure that one year later you still match the Diamond PLANETS balance requirements from February 1st to March 1st, 2024, if you wish to receive those initial Diamond bonus rewards - otherwise they’ll be lost and burned.
To put it simply, in order to claim your HODLER rewards in 2024, you must have the same PLANETS balance you had on that exact day in 2023 (or more).