I would try the following: make a backup of your iPhone, so you can put everything back if the worst comes to the worst. If possible, synchronise DTTG with a different device so you could put the data back easily without restoring your phone). Reinstall DTTG 3. Start DTTG 3 so that your data is copied from DTTG 2 to 3 (use the trial period to do this). If you now delete DTTG 2, the data associated with DTTG 2 will be kept on the device by DTTG 3 for 4 weeks (don’t quote me on the 4 weeks - I think it was 4). Now go to the purchased section in the app store and download DTTG 2.7.9. I would expect that when you now open 2.7.9 your data will still be there (and will now remain there until you delete DTTG 2 again). You will not now be able to purchase DTTG 3, because no purchase options are available during the trial period. But you should now be able to purchase once the trial period ends. You can then delete 2.7.9.
If this were to fail, you can restore your data from the device you synced to; or by restoring your phone.