Not always.   If they happen to be canon to canon, then a much better chance.  I had a 400 sigma that worked just fine with my canon A2E film.  Bought the digital 40D and found they were not compatible.  It needed a chip upgrade.  Sigma had done that for awhile but were now out of chips.  I sent it to Mray to see if he could use it and it wouldn't work on the 20 either.