Kanata,
This is MERELY an opinion of mine ...
Anyday, having a lawyer to facilitate the GC processing would be advisable.
But, I think, at this point, it depends on how straight-forward a case yours is and on whether you still maintain good relations with your previous employer and / or lawyer. If the relations are still good, you could just have them forward the papers as the official approval notice goes to your lawyer (you only get a courtesy copy). On the other hand, if the bridge has been burnt, it is better to have a new lawyer. This is because, though the previous lawyer is obliged to give you your case file at the termination of your retainer for his/her services, he/she may not do so seamlessly! It is here the new lawyer might come in quite handy as an official letter from one lawyer to another will always have more weight than that from the client.
On the other hand, if you expect no RFE (though one can never be sure) and yours is an open and shut case, then you can try to see if you get lucky and not hire a new one. However, I believe you will have to inform INS about this change in representation (previous lawyer to yourself or previous lawyer to the new lawyer) and about the change in job, invoking AC21 etc etc...
If you try to go the second route, I think you have a good 60 day period, in case of an RFE, to go look for a new lawyer.
Please check with others on the forum, who might have been in a similar situation as yourself.
Good luck.