hal12b - 9:45 pm on Feb 24, 2011 (gmt 0)
There is a lot to consider here. If they have pages ranked in the search engines or other websites link to them, affiliates, there can be a serious impact (loss of traffic/sales/ etc..)
I would really ask if it needs to be upgraded first. If so, tread lightly. I am sure it is more than just renaming file extensions for the most part. If there is a shopping cart in place, it may need to be replaced.