Unsurprisingly, I've got the same 'information' on my account page. Thinking about it, having the older/previous figure does normally flag up the magnitude of a price change but it being superseded could be more clearly expressed by saying "until [date]" like they use "from [date]".
However, quoting one amount for the date of my next DD, then another 'taking effect' days later clearly suggests that that will be the amount of the ones after that. It's sloppy.
Sure, both amounts are below the current monthly payment taken from my account but that will not be the case for everyone and utterly ignores the situation of people that need to budget carefully and shouldn't have to factor-in an unclear/uncertain automatic deduction that could 'bounce'. In short, as you said, it fails to deliver exactly what Eon promote as an advantage of the DD system... stable predictability.
This is getting ridiculous!
Why is there no option to say "No thanks, I'd rather keep payments as they are"? All this decrease/increase plays havoc with financial planning.
I understand that this would help a lot of people, short term, but as I can weather these increases, I just want stability, not all these see-sawing payments.
I've pointed out problems with Eon's automatic DD 'calculation' system previously. Basically, it ignores existing credit-balances and if the website won't accept your self-adjustment (via the 'Edit payment amount' feature you get to from 'Change amount', after selecting 'Manage my payments') you have to contact customer services. The obvious conclusion is that Eon prefers its customers having to use their own time/effort, and add to the support staff's workload rather than carry out simple, one-time fixes to their automated billing... as a matter of policy, that Ofgem tolerates.
Hope you can use the website to raise the actual amount taken back up to your usual figure, but expect more hassle readjusting it downwards in the Spring even if so.