Do you have a traditional meter or a smart meter?
If you have a traditional meter, where you top up with a card that you charge up at a shop, then that will continue after you switch to Bulb. You’ll get a new Bulb topup card to use with your existing meter.
If you have a smart topup meter, Bulb do not support those and so it will be remotely reconfigured to a pay monthly meter on the date of your switch. You’ll then need to submit monthly meter readings and pay by monthly direct debit.
Do you have a referral code for £50 bonus when you switch? If not I can send you mine