Hi Andrew,
Our notes to convert AP to Multicurrency.

To convert from AP Single to Multicurrency, we confirm version compatibility, and then we install the MC module.
We ensure all batch data has been posted and retrieved, and posted to the GL. We print relevant balancing reports in AP for the open payables, to balance post MC conversion data.
We backup the data to our cloud server (including currency tables), and run DataCare on the data to be converted to ensure the data is clean to begin with.
We add historic rates to the MC file with at least Month End rates and a rate for the conversion date, and turn on MC in the AP on the conversion date. Within MC, we add the MC codes and rate types.
We use our Gridview RW and we create views into client AP data, to the Banks, Control accounts, Vendor master, AP transaction details, AP transaction history, AP bank Rec Headers, AP Bank Rec Details, AP posting journals, AP check registers, AP tax tracking, and company profile.
We convert GL to Multicurrency, and define revaluation codes in GL.
We add the realized and unrealized Gains/Losses accounts to the GL in home currency. and change/create a USD AP control account in GL with Multicurrency with defaults set to USD, with defined MC revaluation codes (in GL). We change the GL Foreign currency bank accounts and loan accounts to MC with defined MC revaluation codes, and revised default currency codes.
In AP, prior to conversion to MC, we transfer any foreign currency vendors (currently at par 1.0) to new control accounts before turning on MC in AP, and balance the sum of the new control account subtotals to the previous data (unconverted). You cannot change the control accounts on any AP vendors after MC is enabled in AP.
We add currency codes with GVRW to the control accounts, and the banks with the necessary new exchange gains and losses GL account(s).
We filter out all vendors (by control account) and their related transaction activity, AP bank Rec transactions, and convert all of the defined foreign currency transactions leaving the source as is, and manually translating (with XLS) the foreign currency transactions to home values for open transactions while adding new Source Currency codes to relevant accounts, so that the data looks like as if a revaluation was done on that date to change the exchange rate from 1.0 to the actual rate on that conversion day. Completed foreign transactions are left with the transaction exchange rate of 1.0 but all foreign account transactions are updated with the source currency code associated with the foreign currency vendors.
We balance the open payables in both home and source to the previous unconverted transaction reports, and run DataCare on the converted data to rebuild the home values on the AP vendor master, to ensure the data is clean. There will be errors reported but DataCare effectively corrects any anomalies.
We then create the J/E in the GL to balance the Source and home values in GL and balance to open payables, and revise source currency amounts in Banks from home to source, with offsetting entries to Exchange.
We then backup and test the AP data, with another exchange rate to ensure the revaluation is working properly, and that data entry for payments, manual checks, and invoices works as expected, before returning converted data from our cloud server.
Existing data at the client site is backed up on site before we replace with the converted MC data.
We backup conversion data throughout the process on our server to enable easy restart points.

If we have to integrate AP with PO's, X-Company, LInk2Points, and/or BankRec, AR, it is a little more complicated (but completely doable), because you have to ensure the integration is correct between all related modules.

If the client has home currency set to the infamous DOS, we recommend that the client acquire Dakata's toolbox, and change the home currency to something more meaningful than DOS. It really helps for document reporting.

Best,
Brian