Can VMD change its cartoon representation for secondary structure according to trajectories?
As Martin pointed out, VMD doesn't update secondary structure content over the course of an MD trajectory. You can easily do this in VMD by using the SSCache script:

http://www.ks.uiuc.edu/Research/vmd/script_library/scripts/sscache/sscache.tcl

I believe it ships with the newest versions of VMD, actually, but I'm not exactly sure; even if not, you can easily source it:

Extensions > Tk console (in the console, type source Downloads/sscache.tcl (or where ever you've saved the script)). Once you've sourced the script, the easiest thing to do would be to load your topology and trajectory and (in the Tk console) issue the command start_sscache molid, although I advise having a look at the examples and options in the script.