My understanding is that it can take around six weeks for control of S2 Meters to be transferred between suppliers, however this is based on when the SMETS Commands that initiate the transfer process are sent to the meters, rather than when the supply is moved over. If E.On didn't send them at the same time your account moved over, it's likely to cause additional delays. Unfortunately, even with my knowledge there's nothing I can do to speed this up.
There's also no way for the receiving supplier to initialise the transfer process either, because that would otherwise create a huge security flaw where almost anyone could theoretically seize control of Smart Meters randomly and without the real supplier being aware. For this reason, the SMETS Commands to release control and start the transfer have to be issued by the relinquishing supplier because no-one else has the ability to authenticate them. The timescales for this can vary wildly depending on the supplier, but I wouldn't be surprised if there's a huge backlog being generated by the mass-migration - especially because there are unrelated migration projects across all suppliers to transfer all S1 meters to the Data Communications Company (DCC) right now... And there's probably over 10 million of those being migrated over several years. It takes a long time to do all that number crunching.
Because E.On Next is considered to be a "different" supplier to old E.On, all their meters have to go via the full transfer process - there's no possible shortcuts to speed things up unless a supplier simply rebrands and changes their own name while keeping everything else the same. Which isn't what's happening in this case...