If you switch bookkeeping software, or switch from a manual upload to an automatic sync, you need to select a start date for the sync so that the data in the previous yours is not overwritten by the sync.
Go to the Edit client page and select the 'Sync' tab. There you can click on 'Switch sync':
On the next page you will need to choose the first option:
On the next page you will be able to choose a transfer date for the sync:
The transfer date can only fall on the start of a new financial year. It is not possible to switch syncs on any other date.
If you want to switch from manual to an automatic sync:
Switching from manual to automatic goes much the same way as switching between two syncs.
You need to go to the 'Edit client' page and select the 'Sync' tab. There you can click on 'Set up sync':
On the next page you need to choose the first option:
On the next page you will be able to choose the transfer date:
Again, you can only switch the sync at the start of a new financial year. You will only be able to select the start of a new year in the drop down below 'transfer date'.