I'm not entirely sure how H1 transfers work... It shouldn't have effect on your GC though, because H1-B is considered dual-intent status.
The only reason you would like to still maintain H1-B is if you're afraid that your I-485 maybe denied. Also EAD gives you freedom to change jobs ( note that there's limitation there too - you you have to stay with the limits of filed I-485 ) - you may want to read
http://www.immihelp.com/gc/aos/eadap_h1.html for futrher information.
Overall, your situation is tricky, because the AC21 180 days law actually says that I-485 maybe reused for different employer if the application has been pending for 180 days. Now, as helge pointed out, your application
will have been pending for more than 180 days before INS opens it, but this a somewhat loose interpretation of the law. It is entirely possible that it is still a valid one, but AOS is a discretionary process( which means that opinion of the adjudication officer means quite a bit and that any application can be denied at his/her discretion ), so there's dangers there.
I'm not trying to scare the heck out of you, neither am I saying that your situation is hopeless, but you need to evaluate all of your options before proceeding. Talk to a lawyer - you don't actually need to hire one to handle all of your immigration matters - just ask for advice and take it from there. It's worth the money.
You should be concerned with following issues:
1. Your I-485. Make sure that whatever you do doesn't lead to possible denial. AC21 seems to be on your side, and sometimes INS doesn't ask anything or issues any RFEs, but relying on dumb luck is dangerous. Just know your options.
2. Your I-140. Make sure it's not revoked by your current employer. All AC21 business ends if I-140 is revoked - at least it's my understanding. Now there's not much you can do about it, but just keep that in mind.
3. Your current status. Don't accept any unauthorized employment, it mau enable INS to deny your I-485. Employment gap is better then 10 year US-entry ban.
4. Talk to a lawyer. Know your options. ( I know, I sound like a safe-sex booklet

, but the message still remains )
May the force be with you

Good luck