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Public IP address blocks are related to AS numbers. When you connect to one ISP, they provide part of their address space for your usage. If you want to multi-home, you can't advertise the provided addresses from one ISP as also belonging to another. If you get multiple ISP specific address blocks, you can NAT your internal addresses using the ISP provided address blocks, but true multi-homing requires your own address block and so your own AS.
One you have your own AS and public IPs, you can easily change ISPs and/or change how many you "peer" with as now you "own" the AS and IP address block.
