Don't panic because in many cases the issues can be resolved without having to abandon your business' identity. Your mark may not actually infringe on the claimant's mark, you may have a common law trademark giving you the right to use the mark in your geographical area, or the claimant may be willing to license use of the mark, place some limited restrictions on use or request some reasonable modifications to the mark. Remember that pursuing trademark infringement claims is an expensive endeavor and it is often economical for the claimant to work out an agreement with alleged infringers. Usually, a San Diego trademark attorney can assist in resolving the dispute for a reasonable cost. Of course, in some circumstances the claimant will not walk away so easily. It depends on the size of the claimant's business, the strength and recognition of the claimant's mark and the geographical scope of the claimant's customer base. A business won't get very far opening a hamburger joint called McDonald's and utilizing a big yellow "M" at the forefront of its logo's design. McDonald's will pursue a trademark infringement claim no matter the expense.
Don't ignore it because some companies will pursue their claims even if they are not as big as McDonald's. Once a trademark infringement claim is filed in Federal court, you lose significant bargaining power. Even if you capitulate and agree to change your business' name, the claimant will almost certainly want to be recuperated for bringing the action including attorney's fees and other costs associated with filing suit. With or without an attorney, open the lines of communication.
If hiring a lawyer is not economically feasible, start by calling whoever sent the letter and ask them whether there is room for negotiation. However, be careful about making any admissions, especially in writing. For instance, your case will be significantly weakened if you send a letter to an attorney for the other side admitting that your mark infringes on theirs and apologizing for the mistake. Instead, let them know that in the interest of avoiding legal fees and litigation costs you are willing to work something out. Ask what it will take. If you feel you have a strong position, tell them why - your mark has been in use in San Diego County for 10 years well before they obtained their federal trademark registration (giving you a common law trademark). Or suggest that the two marks really aren't that similar and there isn't any likelihood of confusion. If your position is weaker, suggest alternatives that won't involve reinventing a marketing campaign. Maybe the design of your website is too similar to theirs requiring a simple fix. Maybe they'll be willing to license the mark for a nominal fee given that the marks are being utilized in two different states. However, it's best not to play attorney with these folks. Be conciliatory so that they understand your willingness to cooperate. If at any point it seems like you are in for a battle, that's probably the time to contact a trademark lawyer regardless of the cost.