Enrolled owners still pay the Eplus fee. It isn't free. However, if you don't have Eplus you would still get unlimited free Marriott to Marriott retrades. For your new request, are you seeing the button to add EPlus instead of the button to do an EPlus retrade?
When I log in and start to modify the request, it still has the box checked to purchase eplus. However, at the bottom of the page, it states it was previously purchased as follows:
Previously Purchased Items for this Exchange
E-Plus
Change your destination, resort, unit size, or travel dates up to three times for one low fee. More Details
I didn't provide a credit card when I entered the new request (since there is no fee for exchanges as an enrolled member). I'll have to check that I get my refund for the prior on-going request.