yeah it might be a good idea to reinstall all updates everytime a new one comes out.
if you make any changes you could make duplicate.
in 'GPSAVES\Carsets\cmu' you could create your own update. so if for instance you replace the team11 textures (\2005\Data\Cars\Team11\*.tex)
then in also put those files in: GPSAVES\Carsets\cmu\2005\Data\Cars\Team11\*.tex
then you can create an update file from CSM's menu.
so you'd install all the 'official' updates, and then your one. theoretically it should be all ok, but who knows ;k
ps. do all that at your own risk, and ignore it if you're a bit thick :p
____
Tony