With due respect to the advise posted previously,
I think you have received a Cease and Desist notice asking you to stop using the previously registered trademark. (okay registered in different class.)
and that you have incorporated a company (We call it as Trade Name) with the similar name.
The advise would be..
You weigh your self how you have placed, compare with the reputation and goodwill of the notice/complainant weather you can justify that you are not encashing your sales upon the complainant, if yes then go for changing your trade name. (here I take that you are using the mark only in trade name and not on product.)
Defending the Case of Infringement/Passing off will cost you like anything and it is not two slice bread but two slice diomond.