I've been doing a little digging around and it seems to the best of my knowledge the opening poster has got the wrong end of the stick, having recently noticed somethig similiar on my bills i contacted E.on and spoke to one fo their call centre agents who surprisngly was very helpful and informative.
Current customers of the E.on energyOnline Extra Saver are probably on version 1 - 4 and in the T&C's of this product there is a guarenteed time limit of the online discount. So custoemrs seeing this discount decrease, it is more than likely they have reached this expiry period for the version that they are.
New customers to the EnergyOnline Extra Saver product are being put on the Current Version (Version 5) which obviously still has the enlarged discount. I would imagine that anyone on the online products versions 1 - 4 will automatically be transferred to Version 5 as and when their product expires and of course you could always pre-empt this by changing across to version 5 now! From what i have seen on this site and even from E.ons own website there is nothing that will stop you from doing this.
With regards to the telephone number change isn't it just fo all non-sales related contact that there is now an 0845 number? and isn't this much the same for a very large proportion of service providers? Its the same for my bank, my internet and even my gym. I use a website that lets you type in an 0845 number and will tell you if there is a regional number you can dial isntead so if you have free phone calls to these numbers this site will be useful. (not sure if i'm allowed to mention it here but i'm sure if you google it long enough you'll fidn the site i mean)
Something to think about is a free phone number really free? or do you think that a company such as E.ON would pass the cost to them for the provision of the free phone number back onto the customer somehow! say in the unit rates for the electricity or gas they supply? So infact people that do not have a need to contact their supplier are subsidising and paying for those that do!
What will be interesting is that will the apparent cost saving measures, the removal of a freephone number, the reduction in the tesco club card promotion (which i assume would come to E.on at a cost) and the reduction in opening bours which I would think would reduce the wage cost to e.on (i.e less people working for them, and less overtime payments), will E.on then reduce its prices? Well i won't be holding my breath on that front, they do afteall have shareeholder's wallets to think about.