All depends on the module and their roots on v6, and the other modules dependencies that maybe could have changed from v6(6.1?) to v8. 

This needs a solid code review to be able to use it in v8,

but why not? You could update it adapting the changes, it's have been done before, maybe it's easy as no strong dependencies with Odoo core or could be difficult as for example with a deep integration with Odoo Warehouse Management modules that were rewrited in v8

Ok, So there are no rules to follow especially for this update (v6 to v8)I start programming on odoo, it seems complicated to do it

For exemple :

_columns = {

 'route_line_id': fields.Function(_get_route_line_id, type='many2one', obj='delivery.route.line', method=True, string='Delivery Route Line', readonly=True, store=False),

'route_id': fields.Function(_get_route_id, type='many2one', obj='delivery.route', method=True, string='Delivery Route', readonly=True, store=False),

'route_state' : fields.Function(_get_route_state, type="char", method=True, string="Route State", readonly=True, store=False), 'delivered': fields.Boolean('Is Delivered', select=True),

.....

}

When trying to use my module in v8, I have the error saying that fields has no attribute function