No, you may not change the subscription payment frequency whilst your current existing plan is still ongoing, to change your subscription payment frequency, you will need to wait for your current subscription plan to be near the expiry date then only you may change your subscription payment frequency. However, you will still be able to update (increase / decrease) the number of headcounts in your subscription plan.
If your existing subscription payment frequency is 12 months / yearly subscription, then you will be able to change your subscription payment frequency 1 month before the subscription plan expires. Any other subscription plan besides the yearly subscription plan will only allow you to change the subscription plan 1 week / 7 days before the subscription plan expires.