What is the Delta rule on reticketing a return flight when trying to pay to upgrade the outbound flight only?

I'm flying ATL-LIR August 18-23 and ticketed in U on the way there and T on the way back. Called to try and buy an upgrade to I (BusinessElite discounted fare) on the outbound but stay in coach (T) on the way back. Total fare difference should be about 425 plus 100 change fee. Apparently I have to be reticketed still in T on the inbound flight also which is now several hundred dollars more expensive... any ways around this?

Thanks!