Signing up for a Premium Membership is a donation to help Inside Universal maintain costs and offers an ad-free experience on the forum. Learn more about it here.
If you upgrade to a new pass where that specific date you are attending is not blacked out...you can go in as long as the new pass is purchased. Very similar if you upgrade to the Platinum Pass to eliminate black out dates altogether. If you upgrade to a new pass which also is blacked out the day you upgrade...you cannot enter as you have already utilized your "first visit" and the new pass is also blacked out. I hope that answers your question.