What do I need to look out for when converting 9V example schematics to 12V?
but i am constantly wanting adapting them to 12V for using relays,
  large batteries etc.


Why not make a small switching buck regulator - the circuits newly attached to the 12V battery (via the regulator) will take smaller currents to power them from the 12V.

After all isn't this another valid point - you are moving to bigger batteries to make the circuits operate for longer as well?

The amount of time you'd go to (possibly) to make the circuits work from new supplies would be better spent looking up a buck regulator and implementing it.