I am the KING of updating OS's in the middle of a project. I did it last year (with my very first Premiere project, incidentally) and all went well and here we are this year -- another update in the middle of another Premiere project and no software issues yet! I have had two complete system crashes but I think my logic board's days are numbered on this late 2008 MBP. Also, My Dynex eSATA ExpressCard 34 card has taken a dump with this update. I have yet to give up hope as I believe it stopped working in Lion and then worked with an update or two.
I have some fun AVID Media Composer stuff I am compiling so stay tuned for that!