Yes. My wife wanted to play a fighter, but didn't want to be a dwarf (she said she's short enough in real life already). So I let her be a human or elf instead. She chose an elf, and I let her swap the Wisdom and Dexterity scores (hill dwarf apparently gets a +1 Wis, so I said she was a wood elf to get a +1 Dex instead of Int). She used a longsword and heavy shield instead of the greataxe and I let her swap the chain mail for a chain shirt.
She was a very effective character, inflicting 1d8+7 damage in melee, and didn't get hit very often.
For the next session, I can revise a few things based on what Mearls has said; her hit die should drop to a d10, but she should get a boost on weapon damage with bows and probably long/short swords too. Maybe give her the Guardian theme as well, as suggested.
Of course, I wasn't going to show favoritism to my wife in the game, so I left the mix-and-match open to everyone. It was mostly just race/class combo switches; themes and background generally stayed with the pregen classes.
So the group consisted of:
Wood Elf Fighter (Soldier Background, Slayer Theme)
Wood Elf Rogue (Soldier Background, Lurker Theme)
Human "War" Cleric (Knight Background, Guardian Theme)
Human Wizard (Sage Background, Magic-User Theme)
The pregens include a hill dwarf and a mountain dwarf, which appear to have the same traits but differ in ability score adjustments (+1 Wis vs +1 Str), so it was easy to specify that a wood elf would have the same traits as the high elf, but have a +1 Dex instead of +1 Int.
We haven't actually played D&D in almost a year, we only play every other week at best, and nobody in the group was keen on the idea of playing a few sessions with "throwaway" characters, only to create new ones once the character creation rules packet is released. I figured there would be minimal impact with allowing a bit of swapping. I resisted the urge to modify anything else, at least until we've played a few sessions. I think I might run the next session with group initiative, rolled each round just to speed things up.
EDIT: I also gave the PCs one additional language per point of Intelligence modifier, so my PCs could speak Goblin, Orcish, and Draconic. Comprehend Languages doesn't let you speak the language (and requires you to touch the speaker), and there's no Tongues spell in the playest. I actually thought this was an oversight on the character sheets until re-reading the playtest docs after the adventure, as it's been there in every edition of D&D other than 4e. Oops.