It's OK providing you know the old one is the right length. Bike mechanics replacing chains do this all the time.
Note that the old one will be slightly stretched (or elongated because of wear at the pins), typically by about 1/2", or one link, over it's length, so to end up with the same number of links you'll be cutting the new chain slightly shorter than the old.
You can easily correct for this by lifting the old chain a bit as you move down to keep the links in phase with each other.
__________________
FB
Chain-L site
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Last edited by FBinNY; 05-30-10 at 07:53 AM.