In one word, Yes. It is required that the approved plans have the current ship's name and approval stamp of present calssification society.
However, this may not be a serious observation if the previous classfication society and the present classification society are the member of IACS (internation association of classification societies).
All the members of IACS follow similar classification rules and having the plans certified by previous classification society(also an IACS member) means that it complies with the applicable regulations.
So when I say that it may not be a serious observation, I mean the vessel may not be detained for this observation but it will be an observation (PSC or other third party) nonetheless.
Many time, during the change of class the class would have stamped few copies of these plan. And it may just be that the old copies are in use. We may want to check for that too.
If class has not done that, company need to contact the class and look for a way forward.
If annual class surveys are due in near future, these plan can be stamped by the class during that time including the correction for the change of name of vessel.
Otherwise company can send few copies of all the plans to the class and get these approved with any corrections.