I have installed Team Calendars on our Confluence instance which currently has the old Calendar Plugin v. 2.7.2 installed. (It is not officially supported for Confluence 3.5 but it seems to work anyway). After installation the pages having the old

macro did not work anymore. Even after disabling the new Team Calendars plugin those pages were still broken, I had to disable/enable the old Calendar plugin to make them work again.

To the user, this problem will look like a data loss of their old calendars.

In my opinion, the new Team Calendars plugin should at least be compatible to the old plugin (as many companies are probably using the old one), ideally there should be a way to migrate all old data into the new calendar.