Unfortunately if the migration didn’t go through for whatever reason it is too late (it ended over a year ago) and your only choice is to start over. As far as paid content goes the only things from battle.net still in the game that require purchasing things would be forsaken which a large part is getting vaulted in February with the release of witch queen. If you still want access to that stuff before then there is a discounted forsaken package on the store which will give you access to the raid and dungeon, the campaign is currently free to play until witch queen.