I'm confused by this
Surely there was a period of dual-running? When migrating massive systems like this, you don't simply turn off the SAP services on a Monday morning and rely entirely on the as-yet-untested Oracle: you populate the new system with test data and check it does what you need it to do. If it doesn't work, you halt or delay migrating users and live data until the problems are ironed out. Sure, you won't catch them all - but being able to do something as routine as producing financial statements must surely have been a baseline requirement.
Even then, there's usually a period of parallel running where data is entered into both the known system and the 'new' system. Again, if the outputs don't match - or if you discover a process takes 30 people rather than 3 - then you should delay the disarming of the old, stable, system.
Sounds to me a bit like BCC need to swallow a bitter pill and renew their SAP licenses. Yes, it'll cost a fortune because SAP know they have them with their pants down and will aim for full penetration - but it wouldn't be the first time a major project organised by a public body was badly managed (I give you NHS IT, HS2, any number of power stations and, if the Eye is to be believed, the northern Freeports). Umm...unless they didn't keep the SAP system current - abandoning that system this early in the changeover would be almost criminal!
If I'm missing something, forgive me - and do please help further my education.