Elves in Delving Deeper:
Elves begin as either fighters or magic-users, but can change class between adventures as often as desired. An elf becomes dual-classed when he changes class for the first time, and may thereafter use both the weaponry of a fighter and the spells of a magic-user simultaneously.
The dual-classed character must maintain separate experience point totals for each of his classes, with all earned experience going toward his currently active class. He uses the more favorable game statistics of both classes during play, but cannot act as a magic-user while wearing non-magical armor.
Elves in Swords & Wizardry White Box:
The Elven adventurer may choose, on any given day (perhaps when the moon rises) whether to use the capabilities of a Magic-user or of a Fighter. As a result, the Elf has access to either the Fighter or Magic-user chart for to-hit bonuses and saving throws, depending upon whether he donned steel that day or summoned up the power to cast spells.
Character Advancement: Elves may use either theFighter or Magic-user class advancement charts, to be announced at the start of an adventure. They are typically allowed to progress as high as fourth level as a Fighter or eighth level as a Magic-user.
Weapon/Armor Restrictions: Elves have general limitations similar to those of Magic-users, but are allowed to wear special Elven chain mail while adventuring as a Magic-user.
and the Variant Elf in S&W White Box:
Some Referees may want to allow the Elf to advance as a blend of Fighter and Magic-user instead of switching back and forth. In that model, the following abilities and advancement table might be used instead:
Weapon/Armor Restrictions: Elves would have the advantage of both magic and armor at the same time, so the Referee may limit the Elf to chain mail. Elves may not use shields when casting spells. Elves may not use two-handed weapons (two-handed sword, pole arm) when casting spells.
Why I do these comparisons when I don't have access to my OD&D Boxed Set escapes me, but whatever. I believe the S&W White Box non-variant elf class is truer to the OD&D rules, but I suspect the Delving Deeper version is probably how more groups played it.