Need advice with nav4to5 update

Heys I stumbled some problems on updating navision 4 sp2 database to nav 5 sp1
there is some tables with records that has decimal instead of date-time.
How should I proceed changing them since the old nav 4 sp2 database has several companies and records on these tables.
Some tables like Calendar entry, Requisition line, Production order, production order line etc have this chage on them. Transfering data into excel takes about 6hours per table somehow and the xls filesizes are about 10-20megs.

Any help for these matter would be great