Ah, I see, that makes sense now. Your attorney is completely wrong if you are a London applicant. Once you've submitted docs to NVC then it's only about a week to get DQ'ed (you can check that yourself here - https://travel.state.gov/content/travel/en/us-visas/immigrate/nvc-timeframes.html), then you get added to the queue for an interview. That used to be 1-2 months via London (my whole process from submitting the I-140 to having my visa back was 5 months last year), it's lengthened a bit due to the summer rush but is still only 3-4 months.
Frankly, switching to L1 just isn't worth the hassle, you're likely to have your immigrant visa before that all gets sorted anyway. I suspect the lawyer just wants to be able to charge another chunk of fees for doing a new visa application, and the AOS - but you're in the very final stages of what you've already applied for, so I can't see your company agreeing to pay all that for nothing.