you can't unlearn earlier skills, for example, to unlearn mining I you would have to unlearn mining IV, III, and II first in that order, you have to unlearn the last skill first....
You can learn all the skills, and many people have do so - but the more skills you have, the longer each new one will take. After the first 24, I think (or 37 if you learn the Better Learning tree) each additional skill adds a time penalty. As the devs release more new skills, some players will want to trim down their scope of knowledge so a new one doesn't take two weeks to learn. :)
An example - I have all but the 3 Unlearning Skills and Mining IV. I am now learning AKVII. AKVII was going to take 8+ days when I started it. Now it is down to 6+ days. Mining IV will take me 23 days, 13 hrs., 25 min., 7 sec. Both of these skills are partially learned. I stopped both of them long ago in order to learn other things. Mining IV would have taken 24 days, 6 hrs., 33 min. 7 sec. if I had not partially learned it earlier. Check these times against how long it would take you to see how the times increase.
Unlearning 1 will take 19 hrs., 25 min. 7 sec. I unlearned Unlearning I and II to get the Unlearning Badge, to save a tiny bit of time on learning the other skills and because I want to learn it all and really have no plans to Unlearn all the skills that took me so long to learn. This is my progress since the last reset.
If I spent the time learning something, I don't want to spend more time unlearning it, just so I could spend even more time learning again. Skills aren't released often enough for me to worry about the time penalty.
It's also a good way to correct your strategy in acquiring all the skills. Most players learn a lot of half-hour skills up front, because those skills don't have prerequisites and are satisfyingly within reach. A month or so on, you might find that unlearning four of those (2 hours total) can take days and days off learning some more advanced skill sets - and relearning them later will still take only two hours.
What oscarette said :) I've found that if I unlearn the four tuning skills for the duration spent learning a new skill when it comes out, it shaves days off the time. Potionmaking II was MUCH more manageable without the tuning skills weighing me down.
Other skills to consider unlearning/relearning: Snapshotting, Focused Meditation, and Transcendental Meditation - if you don't have Martial Imagination also.
Skills aren't released often enough for me to worry about the time penalty.
Come back in here when the new skill set is released and let us know if you still feel the same way. Construction is likely to be one of them, and you'll probably need it to get the most of your new customizable house. Do you really want those skills to take weeks to learn before you can take advantage of them? What about the skills to come AFTER this update. How will you feel if the rock tells you some of them will take a month to learn?
I have unlearned all cooking, cropping and AK skills. Food is cheap in this game. I did enjoy the skills while I had them but it's time to move on.