I've seen multiple discussions on this and want to share a successful workaround I've used several times.
Enter the conversion as a stock split, using the number of new shares and old shares. This will adjust all the historic and current share numbers and prices to match the new fund.
Then go to the Securities list, and Merge the two classes into the new one. It will assign the new name/class to the old transactions but the cost basis and history will be correct. Just write a note in the Stock split transaction so you will always know the date it changed.