I have two white shirts. One has sleeves that are only barely long enough, and can disappear up the coast sleeve, if not tugged down. The other is 15 years old and slightly yellowed. Next to each other the yellow is obvious; in its own in daylight, it probably appears white except on very close inspection. Sleeves in the old shirt are perfect.
Which would you say is the lesser of two evils?
The lesser of two evils would be to buy a new shirt. I know that what I said doesn't make sense, but you should still do it. In the meantime wear the yellowed shirt with suits and the roll-up the sleeves on the other one in summer with casual linen or cotton pants.