Can I use an h-bridge as a changeover circuit?
The idea is: When solar controller is active, feed a voltage to the
  h-bridge direction input so that I can select my power source.


H-Bridge circuits typically use power MOSFETs, which have internal body diodes (shown in the simplified circit below as D1..D4). The diodes are intrinsic in FETs, but would still be necessary anyway to steer back-emf current through a motor or other inductive device.

Unfortunately, even if all the FETs are 'off' both supplies are connected to the load via diodes D2 and D4. So you cannot select the source using this method. It will however automatically pass through whichever source has the highest voltage.

Another problem is that the H-Bridge controller circuit is not getting power (normally supplied through B+ and B-). To power it properly you would need an isolated DC/DC converter with output voltage equal to or higher than both the solar charge controller and power supply.







simulate this circuit &ndash; Schematic created using CircuitLab

So all-in-all your idea just won't work. A practical solution for automatic switch-over would be to just use two high current Schottky diodes, possibly with individual FETs across them to get lower voltage drop. "Ideal diode" controllers such as the LTC4357 can be used with external FETs for low-loss automatic switch-over.