Updating Only A Small Part Of A Text String In A Field

Hello all,

I have a table that holds a large amount of text in a field that is the body of the email. For example, it might say something like:

Quote: Email tech support at thisemail@email.com if you have any questions about the results of this test.

I need to change the email address in this field. Using this example I need to change thisemail@email.com to thatemail@email.com; however I do not want to change the other text in that field.

It is also important to note that the rest of the body of the emails stored here is different depending on the email.

So basically what I need is a statement that would look at a particular field, search for an email address, and replace that email address with another one without disturbing the rest of the text in that field. I already checked the w3 update tutorial and the update there is for the entire field.

Thanks for the help in advance!