It is not ideal to switch your Payment Processor while your page is live, but if you need to, you can. Keep in mind, you cannot change currencies.
When switching, you want to make sure that any money that was collected on the previous Payment Processor is transferred into your bank account and not left in a Payment Processor account. SImply disconnect one Payment Processor, and complete connecting a new one right away! If there is any time when no Payment Processor is connected, people will be unable to pay or donate.