Yes, you can re-flash your Solo (901 part number) APs, but if you ever need an RMA you will get another Solo AP (that you will have to re-flash again).
Ruckus recommends to return your APs for Unleashed firmware models.
That said, it's also easier to upgrade a set of Unleashed APs, as you only need to upgrade one (Master AP model), and it will upgrade all other models.