If one switches a city or commune do they lose all their lessons they've gained in their guild of choice in their city/commune or do one get a percentage of that back to learn new and different skills for another city/commune

You'll only lose lessons when you forget skills.

Say you're an Aquamancer who leaves and goes Geomancer, the only skill you HAVE to forget to be Geomancer is Aquamancy, so that you can take Geomancy instead.

You don't have to be part of a guild to be in a city/commune, but there may be player-made requirements concerning that. And a lot of power-using skills need a particular type of power. So for example, a Hartstone going to Magnagora, even if (hypothetically) they kept their skills, they wouldn't be able to forceforest, or sap... because it takes power linked from either a commune nexus, or in many cases more specifically - Moonhart power.

In forgetting skills, how much you lose out depends on whether or not you need to transfer lesson pools. The exact amounts are in HELP LESSON POOLS.

If you don't need to transfer lesson pools, I think it's a 20% loss. If you need to transfer between sibling pools there's another 30% loss. And, if you need to transfer again, back to the root pools, I think it's another 30%.

*sigh* Ok, that's probably really confusing. Can anyone clarify?

I got it perfectly. Thank you