Ok seriously, this is dumb, you should not be allowed to change ACs.
In DAoC (Another Mythic game) there were 4 base classes, now while the names for the base classes differed between realms it basically boiled down to: Fighter, Healer, Rogue, Mage. At level 5, you picked your actual class. Take Midgard (Viking realm) and Fighter/Mage base classes for example:
Base class = Viking:
Warrior (Tank/DPS/heavy armor) Thane (Melee/Caster DPS Hybrid, heavy armor) Skald (Melee/Group buffs hybrid, heavy armor) Berserker (Dual Wield Melee DPS, Medium Armor)
Base class = Mystic:
Runemaster (AoE or Bolt damage and some utility depending on spec) Spiritmaster (Pet Class and PBAoE / Lifetaps
Every single one of the "Advanced Classes" was different and had different mechanics, so no they were not the "same class, diff spec". The same thing holds true in SWTOR. Take my characters base class, the Trooper for example:
Vanguard:
Shield Spec = Melee Tank Tactics = Very mobile, melee / medium range, dps build Assault = Ranged but still mobile dps build
Commando:
Medic = Healer Gunnery = Stationary ranged dps with channel abilities Assault = Ranged but still mobile dps build
Except for the same assault spec, which Bioware did for balancing reasons, the two classes are very different and require different play-styles/mindsets to learn and excel at. Allowing AC changes totally kills that and drives down the average level of skill/ability amongst the player base. Mythic did not allow AC changes back in 2001 and they won't now. DAoC was extremely successful when it came out just like SWTOR will be now.