In order to rebook into 1st class, it will be necessary to cancel the entire itinerary and rebook it, so will be $150 plus $50 plus you will need to be able to rebuild the itinerary

See the OW thread on the US forum for recent experience with being unable to change class without cancel/rebook

I do not think it is worth the risk of not being able to get the whole itinerary back , so would not do it unless you can build an entire new itinerary before cancelling the old one