Hi,
if you really want all your transaction to be done in foreign currency for this year fiscal. Then you can follow the below process.
All the balances need to be move one by one in intermediate G/L Account
For Example:
Vendor balance = 50000 LCY
Create one Intermediate G/L Account (Transfer Account)
Then pass one Journal entry which will credit the vendor and add it to intermediate account this transaction is happening with LCY
Now again transfer back from Intermediate G/L Account to vendor but with Currency code in it.
Please note this process can be risky if you have not consider appropriate posting date or amount. Based on posting date and amount the currency exchange gain and loss accounts will be hitting and before proceeding with posting such transactions make sure you have completed the above setups for Foreign Currency in Business Central.
Also please try to post the above transaction in Sandbox first before using Production.
If my answer was helpful to you, please verify it so that other users know it worked. Thank you very much.