Path of the Trooper
1) This quest starts with Master Gwain in the Kamael Guild at Gludio.
2) Gwain says that you must pass a test to be admitted into the higher Kamael ranks. The test is simple in the sense of "straightforward" but not in the sense of "easy". Therefore you should first undergo additional training with Master Shubain in Gludin.
3) Master Shubain is in the Kamael Guild at the west gate of Gludin Village.
4) For the first part of your training you have to go to the Fellmere Harvesting Grounds north of Gludin and kill Felim Lizardman Warriors.
5) Felim Lizardmen (all passive, but social) can be found a bit south of the port spot. The Warriors, who are lvl 17, should not pose any problem, especially since the quest item acquisition rate is 1/1. When you have 5 Lizardman Heads, return to Shubain.
6) The second part of the test is a bit more challenging. Now you have to kill Venomous Spiders who are, as the name indicates, poisonous. At least Shubain has the courtesy to tell you that you should bring Antidote. The simple variety will do and for the 10 spiders you have to kill 10 units should be sufficient.
7) Venomous Spiders (lvl 17, passive) are living on the top of Windmill Hill. Be wary of aggressive Vuku Orc Fighters (lvl 17) and equally aggressive Arachnid Trackers (lvl 19), which are also poisonous. The quest item acquisistion rate is again 1/1.
9) Everything before was training. Now the real test begins. You are to go to the Ruins of Agony, west of Gludio, and kill Tumran Bugbears until you obtain a Bugbear Heart.
10) Tumran Bugbears are passive but level 23, i.e. yellow to you. In addition to that they are hanging out with lvl 24 aggressive Bugbear Warriors. And to reach them you have to fight your way through aggressive skeletons. So bring Healing Potions. I had to kill 4 Bugbears until I got my quest item.
11) Take the Tumran Bugbear Heart back to Master Gwain in Gludio. He will write you a letter of recommendation for Grand Master Gershwin, who is standing right next to him and will complete your class change. In addition to Gwain's Recommendation you also get 81,900 adena, 228,064 exp (i.e. you gain a level) and 13,773 SP.
12) Talk to Grand Master Gershwin and ask about the 1st class transfer, then listen to an explanation about Troopers. If you click on "Transfer to a Trooper" you will be promoted and get 15 Shadow Item [i.e. Weapon] Exchange Coupons D-Grade.