You may have 2 options if you consider switching to QBO:

1. Run the Condense Data utility to lower your Total Target up to 350K (if you choose QBO Plus) or 750K (if you choose QBO Advanced). Then you will keep using QB Desktop to access historical data.

2. Purchase a 3rd party service to convert the whole data.

QB Online doesn't provide the assembly feature for manufacturing companies. You should have a 3rd party manufacturing app to integrate with QBO.

1. For simple assemblies (https://go.zoho.com/H3R)

2. For complex assemblies (https://psref.katanamrp.com/quickbooks)