To upgrade from another Skyteam program you have to buy the top 2 most expensive economy fare classes. Chances are you can get discount biz cheaper if planned in advance and save your miles/points.

Typically I only see the alliance wide upgrades as an attractive option when either 1) company is paying but requires an economy ticket (remember economy only policies aren't always cheapest available policies) or 2) was rebooked into Y from another airline due to schedule cancellation in advance, etc.