The semi-mount(XR1881) 10k yellow gold ring costs $437.79, and the marquise lab grown diamond costs $244.74. This brings the total price of the new ring to $682.53(437.79 + $244.74).
Your remaining store credit for the original ring is $657.82 (258.32 + $399.5). Therefore, if you exchange, you will still have to pay the price difference of $24.73 (682.53 - $657.82).
Stone: https://www.doveggs-seattle.com/collections/marquise-lab-diamondsmade-to-order-premade/products/doveggs-1-390ct-marquise-f-vvs2-lab-grown-diamond-lkp38a59?_pos=74&_fid=8a9a86a34&_ss=c
XR1881: made without the milgrain and rope design, just plain smooth gold
size 5.5