From your post it appears the scheme will be frozen, not wound up as you say the benefits in the DB will remain. And you don't mention a transfer value. So there will be a deferred pension payable at 65. Unless they are buying deferred annuities then there is a risk that the fund will not be able to pay that out. So John should find more details from the Trustees about the funding position.
There is the risk that the new contribution rate will not buy the difference between the expected pension at 65 and the deferred pension at 65. Having said that, 20% is a very good deal and will likely be 20% of full salary, not pensionable salary ( salary less the integration with state pension). If John had a salary of say 40k with a 40/60 integrated pension then his pension at 65 would be 14.6k/yr with full service. Now it will be 9.9k if all goes well. So 5k down which would cost about 125k to buy. But employer will be giving 8k per year for 13 yrs so even without growth that would be 104k. All very simple figures and ignoring inflation/salary increases etc but I think it shows the key point to look at is how secure the deferred pension is.
If scheme was actually being wound up then I see that the Trustees would be obliged to look to follow the example set by the Trustees in the recent Omega Pharma case whereby they secured a much enhanced transfer value from the scheme for deferred members by putting a demand on the employers for a lower discount rate. Much would depend on the wording of the Deed.
John may also check to ensure that other common benefits within many DB scheme will also be in the DC arrangement, e.g. death in service, spouse's pension, long term illness.