When transferring users from one term to another, you have 2 options available. The first in the Action Manager feature available in the dashboard, and the second is the Piano API call publisher/term/change/do.
You are able to upgrade or transfer users to terms even when the old term does have a trial period and the new one doesn't. It also works vice versa. The length of the trial does not matter in these cases, it can differ and you still will be able to transfer the term.
When you are creating the term to which the users should be transferred to, there is also a checkbox Apply trial period for new customers only. If checked, it will keep the existing users from receiving another trial period on this term.
You can also keep this checkbox unticked and all of the users will have a trial period (if enabled on the transfer to term).
When implementing such a term transfer via the Piano API call /publisher/term/change/do, you are able to configure the variable billing_timing with the values 0, 1, 2 or 3. The differences between these are explained below:
0 means that term change should be done immediately
1 means that it should be done at the end of the current billing cycle
2 means that term change should be done on the next sale date of the transfer-to scheduled term
3 means that it should be done at the end of the current billing period of the transfer-from scheduled term