You didn't mention the original plan size so there's no way to tell exactly what the narrower cab means or how big a difference there is from the original plans.
The smaller box will have less internal volume than the original plan and so the box tuning will be higher from the original.
This might not be a problem unless the modified cab is a lot smaller than the plans call for.
Your 15" speaker will still fit on the baffle board so the sun will still come up tomorrow. You might be able to compensate for the smaller box volume somewhat by adjusting the port size a bit smaller or longer.
Here's a link to some online calculators you might find helpful: http://www.mobileinformationlabs.com...alcu_index.htm
More information about the modified cabinet would be helpful.