Currently, if one owns Toast 9 and the Toast 9 Blu-Ray plugin, there's no upgrade path for the the Toast 10 plugin. The Toast 9 plugin will not work with Toast 10. Thus, unless one upgrades to the Toast 10 Pro version which includes the plugin, a new Toast 10 plugin must be purchased at full price ($20), in addition to the Toast 10 upgrade price.
This may be an oversight by Roxio, which became aware of this, this morning, per Roxio Customer Service. Perhaps a change will be made; but if not, I thought it prudent to make the members aware of this.
Edit: Roxio has fixed the "no plugin with upgrade" issue this afternoon, adding the (free) plugin to the upgrade path for Toast 10. Good for them! Looks like it pays for customers to contact them and politely address their concerns!