score:10
According to the Mileageplus FAQs
Can I receive credit for trips taken before I had a MileagePlus account?
You may receive mileage credit for United-operated flights taken up to 30 days prior to your enrollment date free of charge. No mileage credit will be given for partner flights.You may also receive mileage credit for United-operated flights taken between 31 days and six months prior to enrollment for a $50 service fee.
You could use the miles you earned via that route to obtain free or reduced cost flights potentially. There is no other benefit that would allow you to achieve discounts due to your previous flying on UA.
Upvote:4
Probably not worth it, even if you have still all relevant documentation and are willing to pay the service fee. Over the last two years United has significantly de-valued their loyalty program.