During the trial period, you can use BANDZONE as if you had booked the “Plus” plan. This means you'll face (almost) no usage restrictions - only those that are also active in general and with the Plus plan: namely, a maximum of 32 band members and 8GB storage space.
You can book a paid plan at any time during the trial period, e.g. to use limited-time coupon codes before they expire. Unused test days expire if you book a plan during an active trial period.
After the trial period has ended, all of your band's data will be retained and you can book a suitable plan at any time. After booking, you can pick up where you left off.
You can access the booking page for each band via your band overview https://band.zone/myBands and, in the case of an unexpired subscription, via the main navigation in the band section: