The limitations of changing public IP addresses of CVM instances are:
A CVM instance can host up to 5 websites, and each website can bind to multiple domain names.
Yes, as long as your CVM instance can connect to the public network properly.
If you have reached the maximum number of pay-as-you-go instances that you can purchase, you cannot purchase more pay-as-you-go instances. For more information, see Purchase Limits.
To query the use limits and quotas of CVM resources, see CVM instance use limits in Use Limits Overview.