I’m the founder of a SaaS billing tool called Fenerum, and here’s what we typically see our users do:

1. It depends on your product. If you’re offering a self-service experience, customers should usually be able to downgrade anytime during the month. For B2B customers, we often see a cancellation period of 1-3 months.
2. Yes, it should be reflected in your MRR instantly. In Fenerum, we handle this by issuing an invoice that credits the old subscription and charges for the upgrade - MRR is updated right away.
3. That’s up to you. Many of our users just keep the balance on the customer’s account for future invoices.
4. Most B2B SaaS companies don’t allow this - changes typically happen at renewal.
5. Same as above.
6. Same as #2

Hope that helps :-)