I noticed that when I enter a ten-digit ISBN it returns a match to a thirteen- digit ISBN. I know that usually you just add 978 to the ten-digit but it needs a new check digit on the end. How is that being done, is the software doing it or is it a lookup somewhere?
Book Collector automatically converts the ISBN-10 to an ISBN-13 by putting 3 numbers in front of it. The check digit (last number) which is a sum of formula of all numbers in front of it, changes, because 3 numbers are added.
However, the ISBN-13 number is still unique and means the same as the ISBN-10 number. There is no way to change that back to ISBN-10 with Book Collector, simply because it’s not necessary.
If you try searching one of your ISBN-10 numbers within your own collection, Book Collector should come up with the ISBN-13 number because it converts it again.
The last number of an ISBN-10 is a “check digit”. It is a number that is generated by a formula of all 9 numbers in front of it.
- The last number of an ISBN-13 is also a “check digit”. Because the formula (and length of the ISBN) is slightly different, the last number will change from the ISBN-10 number.
I would advise to leave the newly generated ISBN-13 in your database. Or at least do not change the last digit (as the ISBN will not be right anymore then).
I do not want to change the ISBN, just curious how you translated to the 13 digits. Thx
Gotcha. Basically what our server does is a normal conversion that always translate both sides if need be 
You can try it yourself on all kinds of websites such as ISBN Converter | ISBN.org
Let me know if you have any further questions on this.