I'm not seeing people buy M365 Business w/ Teams and then "switch" to M365 Enterprise w/ Teams.
I think the "upgrade" is intended for customers with M365 E SKUs in place using in annual / long-term (EA) contracts to be able to renew, upgrade (E5 anyone?), add, etc.
I haven't personally tried establishing a new commitment using M365 F3 w/ Teams and then securing a "step-up" to (for example) M365 E3 w/ Teams. That would be curious.
Finally, some customers have month-to-month CSP contracts as overflow. Although I haven't seen it in Microsoft documentation, other sources indicate you can change billing frequency of an existing subscription commitment (monthly/annual payments) but you can't transition a month-to-month commitment to annual as an "existing customer" - meaning you'd end up having to buy Enterprise SKUs/ w/o Teams, then buying the additional Teams Enterprise license if you move to annual.