Hi,

You need to redefine the field to change the field type using inheritance. 

E.g. :
Original one2many field:

class mrp_bom(osv.osv):
    _name = 'mrp.bom'
    _description = 'Bill of Material'
    _columns = {
       'bom_lines': fields.one2many('mrp.bom', 'bom_id', 'BoM Lines'),
    }
mrp_bom()
Do the following to change the field type to float:
class mrp_bom(osv.osv):
    _name = 'mrp.bom' 
   _description = 'Bill of Material'
    _inherit='mrp.bom'
    _columns={
        'bom_lines': fields.float(''BoM Lines'),
    }
mrp_bom()
Hope this may help you.