I've always wondered if I wait too long to Dual Class Imoen. She's 6th or 7th Class Thief right now and I still haven't pulled the trigger. I haven't made it to the city, yet, and I fear I will really want a thief in the city. Once I dual Class you lose her Thief skills for awhile, correct?

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There are already some posts regardig this matter, so I recommend you to use the search function for further information.

And yes, if you dual class her right now, she will be a Level 1 Mage until she reaches level 8 as a Mage. Only then, you will be able to use her thief abilities (including using bows and armor).
So, even if I don't know the numbers right now, I suspect the experience cap will prevent her to regain her thieving skills. (If you don't mod, I think a character can go as high as 161000 XP with TotSC).
Even if it depends on the rest of your party and if you are only playing BGI, I would advise you not to dual-class her. I might be wrong though.

tl;dr

If I were you (asumming you don't have any other thief in the party), I would go for Edwin as a mage and keep her as a thief.

I'll admit I didn't use the search function, but I have never had very good luck with that feature in the GOG forums. Thanks for the links. My thread originally was going to be a poll, but then I decided to see if I was wrong in when I usually do the Dual Class.
I'll probably keep her a pure thief. Dynaheir is just too specialized and can not cast a lot of the spells I want to be able to use. I intend to keep her around with Minsc, though.

You can dual-class Imoen (or any human thief with the required stats) at level 6 to a wizard and she can reach level 9 as a wizard in BG1, the same maximum as a single-class wizard. If you dual-class her at thief level 7 she can reach wizard level 8 (and thus can't cast 5th-level spells - and IIRC most of the worthwhile 5th-level spells are ones that Dynaheir can't learn). If you dual-class her at thief level 8 she can still reach wizard level 8 within the XP cap but, as this would not exceed her thief level, she would never get her thief abilities back.

I've never risked doing the dual-class thing. I just kept her a thief the whole way through BG1. But I hear it makes sense to dual-class her.

I'd say 6th. You can still reach the maximum wizard level and you get Imoen's Find Traps skill up to 80 (as high as it needs to go; if ever that's not enough, Potions of Perception are your friend) with points left over for other skills. You'll need another thief in the party while Imoen rises again, but that shouldn't be a problem; she got to thief-6th round about the time I was able to pick up Coran in Cloakwood.

If you dual her do it at level 4-6 and don't dual to mage dual to illusionist or conjurer.

But pure class is fine for Imoen, you don't have to powergame. It's far more comfortable having someone to handle locks and traps perfectly all the time in your party.

Interestingly enough in Baldur's Gate 1 she was just a Thief, but in Baldur's Gate 2 she becomes a Mage/Thief.

When did that happened, especially in my playthrough of Baldur's Gate 1 I never changed her class she remained a Thief throughout the entire game.

That is done in BG2 for story reasons, so she can be arrested for an unsanctioned use of magic.

She probably did that stuff behind your back and wasn't ever going to tell you she could cast spells.

That just shows, she's a rogue at heart.