HI Adriana,
You cannot enter the account assignments in cashflow again once the flows are posted. However in TPM80 you can manually assign the Funds in field New Fund.
Once you execute TPM80 on second Due date, you will see one line item with your position in fund 5828SA143, here you can assign the fund 5828SA141 in New Fund Field and manually change the nominal amount to 50% of the position. Now if you check TPM13 or run TPM80 in test mode you will see 2 positions, one in fund 5828SA141 and other in 5828SA143.
On the Third Due date when you execute TPM80 you will see 2 line items with funds 5828SA141 and 5828SA143. So in the input screen, enter 5828SA141 and execute and assign the fund 5828SA143 in New fund and complete the transfer. Now if you see TPM13 or run TPM80 in test mode again you will see the whole position in fund 5828SA143.
Regards,
Nikhil