Looks like the upgrade requires a new main board so my guess is you'd have to upgrade the whole fixture. At least with LEDs you don't have to worry about the cost of bulb changes, get reduced heat, etc. That was one of the reasons I made the switch . . . I was looking at buying 8 x T5 bulbs once or twice a year on my old tank which would have been at least $200 each time and it helped keep my tank cooler. You could also just sell the the Sols later if you decide to upgrade to something else which is the route I went. Quality lights should always sell . . . it is just a matter of price point but at least a lot of folks are jumping on the LED bandwagon.
If it makes you feel better, it costs even more to be on the bleeding edge to keep up with the newest stuff. If you bought 5 Hydras you'd be looking at $2000 or 5 Vegas at $2500. Now that they have additional colors, I'm looking at getting new Power Pucks to add violet LEDs and I want the new style Hyrda lenses with the diffusers. Problem is, the new style lenses do not work with the original Vega white LED (the LED is too large and the Hydra lens would crush it) so I'm considering upgrading (i.e. replacing) all 20 pucks at $50 ea = another $1000 to make my Super Color Vega fixtures. At least with used Sols, you can get in at a lower price point, see how things go, and then decide if you really do want to upgrade. If you do decide to upgrade, you still have some value with the Sols in that they can be either upgraded or sold.