Marketable title is actually a different concept which is not the same as its plain meaning in English. As to your questions, yes you should have a lawyer prepare a modification of the easement. As to who pays, perhaps if you split the costs by agreement with your seller that might be equitable.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.