When I export my email data, will it be deleted from my Comcast.net account?

No. Saving your Comcast.net email data – whether by downloading it or exporting it to another email provider – does not delete your email data from your Comcast.net email account. However, all Comcast.net email accounts that are not moved to Yahoo within 120 days from when the Yahoo terms of service become available to the user will be deleted. 

Any email data will be removed from Comcast systems 120 days from when the Yahoo terms of service were made available to the user.