Pippa, you can still use the old Kobo app. You need not install the update. That being said, one of the problems with the older version was that sync was for some unknown (to me) reason not working properly on the old Kobo App for iOS. I'm assuming that sync was fixed in this version. I haven't heard about it yet from my iOS friends
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More then that, the App is fairly frequently updated for new features, which, if you do not update, you will not get. There are alternatives to in-App purchasing, so that might be the best bet.