lab machines do not read profiles, they use the actual "numbers" of the colors to print. Profiles are like translation tables to explain the numbers.
hence if you have a calibrated monitor, that last step you do is to convert to the printing profile, which will change the actual numbers of the file to compensate for the printing. Hence when you changed it to the printing profile, the colors may change abit wacky.
Please read the above site and DPReview's Pro forum carefully.
The profile thing, sigh . It does matter. Only if your image is in Lab mode (aka L*a*b) does it not matter. Do not convert to the destination profile; that is totally wrong! :nono: . Doesn't anyone understand what different color space are and how to use them? :mad2: ;(
What you need to do is have the profile and under View->Proof Setup, see the image with the destination profile with gamut warning on if you want. This will 1)tell you what it will look like if you print it out, provided the profile is accurate 2) warn you what parts are not printable and give you a chance to make the changes yourself.
Please read the books I had recommended on colorspace before; use the search function.