Apperently, it was in the cards for a while to close the original and have a smaller version of the show open a couple of years later. I think that they did this because they were spending a lot of money on running it at the imperial which they didn't need to do. So, they closed it and opened it at the smaller Broadhurst theatre.
As for the reason why the orchestrations are different is also because of money. Broadway theatre orchestra pit sizes are determined by the number of the seats in the house. The Broadhurst has a few hundread seats less then the Imperial so the maximum number of people that could be in the pit is less. However, on broadway no matter what theatre a show is in producers always go with the minimum number of people in the pit to save money. Les Miserables was no different however they couldn't lower the number of people in the pit and have it sound the same so they had to change it. They made the pit at the Imperial as small as they could to save money for reasons I just said. They couldn't possibley make what they had any smaller and still work so they had to re do the entier thing.
"If you try to shag my husband while I am still alive, I will shove the art of motorcycle maintenance up your rancid little Cu**. That's a good dear"
Tom Stoppard's Rock N Roll
