Switching Multiclass and Dual Class [AD&D]

A suggested, but not playtested, change:

Humans can multi-class, demi-humans dual-class (1e: Character with two classes)
The rationale is this: Humans, being short lived, try to do everything at once. Demi-humans, being generally less time-limited, will take the time to learn a specific trade, then move on to another. However, to accommodate this idea, I'm going to dig up some things and toss them around.

Human Multi-classing:
Humans can multiclass in any valid combination (ones not restricted by alignment).
However, humans MUST have excellent stats to do so... a 15 in one class's Prime Req, and a 17 in another (being really talented, they can do both). This will necessarily limit some multi-classes, and, in 1e, will necessitate adding PRs to assassin and monk.

Demi-human Dual-classing
Demi-humans need only minimum stats to dual-class; changing from fighter to thief needs only a 9 Strength and a 9 Dex.
Dual-classing cannot happen before 3rd level; you must reach 3rd level in a class in order to leave it.
Dual-classing embargo on previous class abilities is shortened to 4th level in new class, regardless of previous class's level.

So, for example, a human who is going to be a fighter/mage needs a 15+ in Strength or Intelligence, and a 17+ in the other. If they want to be a Druid/Thief, they need a 15+ in Dex or Wisdom and Charisma, and a 17+ in the other. In 2e, this favors specialist wizard/thieves a little bit... any transmuter can be a multiclass thief, if they want.

For a dwarf fighter turned cleric, he needs a 9 Strength and a 9 Wisdom. He might go as high as he likes (up to level limit), then switch classes. So, he might go to level 7 as a fighter, then start over as a cleric. Until level 3, he would have to function exclusively as a cleric; at level 4 Cleric, he'd be able to use his old fighter abilities.

Why a limit of the embargo to through 3rd level? Partially to allow unusual dual-classes (where a low level limit might restrict ever getting the benefit), but also because the long embargo never really made sense to me. I'm already an expert fighter, and I can understand having to take time to practice... but I'm really good at fighting, so I can probably work it in there without spending forever as a rank incompetent.