Geomancy is the art of interacting with the flow, and thus geomancers can minorly influence what changes, though usually only locally, and only within the past few rounds or minutes. For example, a geomancer could cause it to slick from a recent rain, but would not be able to change who ascended the throne at last year's inauguration (though something like that could be affected by a quake).

Geomancy involves things you could have done, such as cleaning a room, juggling a small object, or finding a needle in a haystack; things others could have done, such as agreeing to a trade, tripping on a bootlace, or surrendering during battle; and things that could have happened, such as lightning having struck a particular target during a thunderstorm. Thus, sorcery becomes an art of exploiting circumstances to your advantage.

Since geomancy is tied to the earth, it will be mildly stronger near a ley line, mildly weaker when far from one, and especially potent near a nexus. Since the nexuses have healing properties, it will also be possible to heal using geomancy, to a limited extent.

Mechanically, you will work with the GMs to invent appropriate, unique geomancy effects for your character. You will have a small number of known effects similar in number and power to other spontaneous casters. There will also be an "improvisation" system to conjure custom-tailored magic, but it will be ritualistic, requiring a longer period of time, and generally not be possible during combat.

It might be possible for a geomancer to have a power that could only be invoked during a quake, to have some small influence over it, but we would need to discuss in more detail. And the merits/flaws system also allows this sort of customization without necessarily taking levels.

Keep in mind that from your perspective, it is always everything else that changes—you and your memories stay the same. So as a geomancer, from your perspective, you would be altering your surroundings such that those around you no longer recognize you, etc. Changing gender is difficult, changing race nearly impossible, but changing clothes or physical appearance (especially scars, hairstyle, etc.) isn't too hard. The rule of thumb is: the more extensively your past would need to change to make you appear differently, the more difficult it is to do.

That said, functionally you could change your character build as a consequence of using your abilities. You would just need to play the character as always remembering being the way he/she is, because as always with the flow, from his/her perspective it was the surroundings that changed. Historical relativity!

For example: for minor thieving type powers done with geomancy, the effects could be along the lines of "what if someone had forgotten to lock this door" or "what if that person agreed to trade me for that object" or "what if the object I'm looking for has been placed in this safe." (Alternately, rune magic would work fine as well—you could use existing spells like knock, open/close, telekinesis, etc.)