DOFD won't change unless you made payments and brought the account current. At least that is my understanding. Is your credit report showing DOFD as 3/18?
Do you have documentation of when the account with Comcast first became delinquent (such as old bills), and that it remained delinquent up to the date it was referred to the debt collector (which will be the Open date of the collection)?
DOFD is, by definition, the date of first delinquency that immediately preceded the assignment of collection authority, and not the date of assignment of collection authority.
If you have documentation, file a new dispute contesting the accuracy of the verification of the DOFD in the prior dispute.
You must include new documentation in order to get a subsequent dispute on substantially the same basis considered.
Alternately, you could proceed to filing of civil action under FCRA 623(c) since you now have a verified dispute with unreasonable investigation of its accuracy. However, filing a renewed dispute with new documentation would be more prudent.