I use an app which was running in OC8 and continued to work when I upgraded to OC9.
However when I do a completely new install of OC9 and place the app contents in the apps folder, it no longer works saying it needs OC8.2 to work.
Clearly there is a check of some type that does not occur in the upgrade, but is done on a new install when checking the apps folder.
Is there a file which is present in apps to constrain the versions it will work in - if so is the file/location common to all apps?
Is it the tasks app?
Thanks for asking tflidd.
It was in fact 2 apps, the Tasks+ and Calendar+ apps.
They were not supposed to work after OC8, but I have got them both working now in the latest version (9.0.4) by simply changing the 'max-version' in the info.xml file in the apps.
However, although they seem to work perfectly on the server they do not synchronize to the PC or phone - so I guess the coded Caldav syncing is not compatible.
Yes, both apps are not just for fun marked as incompatible with oC9. 
The whole CalDAV and CardDAV interface was moved from the Calendar and Contacts App into the ownCloud core itself. The "new" Calendar and Contacts Apps are now only CalDAV / CardDAV clients so the Calendar+, Tasks+ and Contacts+ Apps would need a complete rewrite to make them compatible with oC9+ again.
Shame 
These apps offer some extra features over and above the standard Calendar/Tasks apps in OC that would be nice to keep - but I guess they have to go if it means they don't sink with Thunderbird (PC) or Android (Phone).
If it is very important, you can stay with OC 8.2 and wait with your upgrade until the new calendar/contact-apps provide the necessary features.
I'd say they were more 'nice to have' rather than 'important'.
I guess this is the disadvantage of using systems where content can be supplied from independent developers - if you use their contributions and they decide to abandon them it can be disappointing.