The most common mistake when changing software is deciding to "move everything". Carrying years of closed movements into a new system takes a long time and recreates old mistakes in a new place.
A healthier approach is to pick a cut-off date. You carry what is still open at that date: customer balances, unpaid invoices, cheques you hold, stock quantities and cash and bank balances. Closed history stays in the old program and is consulted there when needed.
After the transfer there is only one question worth asking: do the balances in the new system match the old one? Customer balances, cash and bank balances and stock quantities should agree exactly. If they do not, do not carry on until you know why.
Finally, you should be able to see line by line what will be transferred before it happens. In Kumpara nothing is created until you approve it, and missing tax numbers, duplicates or amounts that do not add up are shown to you first.