I would think that you're subject to the -11g rules, where the increase in benefits must be nondiscriminatory. And another result of the amendment being adopted more than 2.5 months after year end being that you're not going to be able to reflect the benefit increase in the valuation.
I just had something like this, where I grumbled, why wasn't the benefit change just made part of the original document signed last month? Because I think then it would have been okay, because a retro-adopted plan is deemed signed on 12/31.
Willing to be wrong, though....