First backup your petty cash account that you are using then if nothing works we can get back to this point and be no worse off.
Go back to your old pc and create a backup of the petty cash account - to a usb stick or shared drive.
Try restoring from backup on new PC - if this works correctly you will have to re enter info since April.
Another option is to use the Export option on the old PC BUT when this is Imported on the new PC it will give duplicate transactions so before you Import, delete all transactions before April.
For future reference, I use the Archive option on a regular basis to save past info.