There are numerous reasons why a scheme trustee would decide to switch providers from one company to another - better fund choice, more suitable funds, better service proposition etc. But whatever the reason, the trustee must always strive to act in the best interests of the scheme members. I think it would be a fair question to ask the trustee (or perhaps the broker if there is one) to outline in writing or by e-mail what the reasons for the change are.
If the death in service / illness benefits are operated as a separate insurance policy, they can be continued if the employer wants to do so. On the other hand, if they're bundled into the existing pension scheme and paid for by cancelling units in the pension fund, they can't. There's no obligation on the employer to continue to pay for these benefits if they're not in the employees' contracts.