I had this problem with my student loan. The original SL had a date opened of March 2001. It was purchased by a different provider back in 2011. A few months later it showed as two accounts: one closed in 2011 and another opened in 2011. So the new one had (from my point of view) the wrong date opened and also failed to show the correct dollar value that the loan was originally for.
It took a lot of phone calls between me, the original loan handler, the new loan handler, and the CRAs. But eventually I got everything I wanted. Eventually means "after six months and many calls with all the players."
You are right that if the new open account fails to show that you have paid off a huge chunk of the original loan, it will harm your score for the first year or two. As you get closer to paying that off, however, that score harm will go away. And then when you have NO open installment loans at all (everything completely paid off) your score will take another significant hit (ironically). But we can tell you a method you can use to prevent that, so that you keep all the benefits of having a loan that is almost but not entirely paid off (which FICO loves).
