In 2015, we updated the OALD app in order to be able to relatively simply migrate existing users from OALD8 to OALD9 (the new ninth edition of the OALD). The updated app includes 100 free sample entries from OALD9 so customers have the option to preview it before deciding whether to upgrade.
Of course there is no obligation to make the upgrade to OALD9 and existing users of OALD8 can re-download and continue to use their OALD8 for free from within the updated app. Instructions on how to do this can be found here: https://oxfordeltdictionaries.zendesk.com/entries/108384686-I-already-paid-for-OALD8-I-don-t-want-to-pay-again-to-upgrade-to-OALD9-How-can-I-keep-using-OALD8-fo
The update also allowed us to change the sales model of the app for the benefit of new customers: the app is now free to download and, as mentioned above, contains 100 sample entries from OALD9 - this way, customers can now preview the dictionary before deciding whether to pay to download the full version (via in-app purchase).