That would mean PRC lied when they showed "DF-31A" in 2009 parade and Kristensen was right when he suspected that it was just a bit modernized DF-31 (CSS-10 Mod 1).
Well, I don't know this Kristensen but I doubt he can read Chinese or have easy access to technical literature published in China, otherwise he won't be suspecting something immediately obvious in official publications. I don't blame him since most western analysts fail to meet these criteria.
The improvements of DF-31A over DF-31 include: replace NTPB propellant with N-15 (equivalent to NEPE-75 used on Trident), replace steel first stage casing with synthetic fiber, second and third stage with C-C composite, improved penetration aid, etc. IIRC the overall dimension of the missile did not change.
As to the source, I can offer a bibliography if one can read Chinese. Those are the books you can't find in a general bookstore. Research libraries in leading universities in China is a good place to search, but I can promise you that none are classified and you won't be asked for security clearance.
It's not like the USAF space command or major aerospace contractors such as LM and Boeing used to publish detailed specifications of their strategic missiles. The information available in the public domain today came from the hard work of generations of space historians, who spent decades digging in the mountains of declassified documents.
Unfortunately the same cannot be done on Chinese rockets. Apart from the obvious lack of interest, the main obstacle is always the language. The 'top secret' sensation is largely due to the inability of most western technology analysts/journalists/historians to master the Chinese language, which prevent them to do bibliographic research in a professional way. Search engines and machine translate does not give you quality research or quality journalism. It doesn't help when their Chinese acquaintances are often amateur blog writers or casual observers, mostly ill informed in serious rocket science and have very little real insight into the country's defence industry.