Actually the software is exactly the same. The on-line version runs on cloud servers in reputable datacenter while for On-Premise we install the software packages on your own server
Yes, you can switch at any time since the software and database are exactly the same.
For the Online version you pay a monthly subscription fee which includes hosting, maintenance , and support. For the On-Premise version you actually purchase the software at a one time cost, maintenance and support is offered with a monthly fee
The online version does not need any IT staff, and servers are located in a secure data center. The software can be accessed just by using a standard web browser