Hello,
1) Once you switch BCS (on year basis), you cannot use Former. You can do the switch in the middle of the year
2) There is nothing specific about implementation in the middle of the year. You will have to migrate your budgetary documents of this year to BCS
3) Theoretically, yes.
4) In SPRO you have a folder of BCS migration activities + quite exhaustive description on help.sap.com
Regards,
Eli