Fine if you're young but what happens if they do that to you when you're in your late 40's? I doubt you could afford to make up the shortfall in time. As far as you're concerned you've done everything required to ensure you get a decent pension then part way through the 'other partner' effectively says "we're not going to keep our side of the bargain, sorry about that. Tough luck for you though eh?". It shouldn't be allowed for an employer to change an employee's pension scheme without the expressed permission of the employee in question. They should be able to ask and make a case for doing so - i.e. "if you dont' agree to this we may go bust and therefore you'll get no pension at all" - but to unilaterally change an agreement part way though is simply immoral and should be illegal.
I could get all class war about this but I won't. It's simply a case of a company (in this case) going back on it's word and that should never be acceptable.
Jon