It should be very obvious in your 2008 paper work.
If it is not, and if you have not already replaced the 2008 version, then call Microsoft and ask them about using the old license. Then if they say that it is OK, ask them how to get the old license directly from the 2008 version. SQL 2008 and 2019 are recent enough that you should expect them to have the information available without your having to pay for the answer.
