When the transaction is first made, the donation information is stored in the billing section of the data base as opposed to the constituent profile record.  As such, if the receipt pulls the email address from the transaction via an S120 tag, the receipt will always show the original email address. If, however, the receipt pulls the email address from the constituent record via an S1 tag, the update made to the constituent record should show up in the receipts of subsequent gifts. The S1 tag for email address is:

[[S1:primary_email]]

Note that if you use either the "Generate Receipt" link or the "Regenerate Receipt" link to generate any new receipts after updating the constituent record, the S1 tag will pull in the most up to date constituent information.