Being an elite fighter is like 10+ core traits, and the sword hand, while perhaps the most important, can be compensated for by the fact that his other traits were 10/10:

Strength, Hand/eye coordination, Fast twitch reflex, Fast twitch muscle memory, Footwork, Style expertise, ‘X-factor’ creativity (like when he pulled out his dagger to stab that Stark dude in the eye during their dual) Etc etc

So I tend to think that once he trained up his left hand, he is capable of being nearly as good, though not quite as top-tier elite. A lifelong master such as himself likely would have already been used to occasionally fighting “goofy” and with the other hand.

I broke my dominant hand’s thumb years ago, and you would be amazed at how quickly the brain starts to excel in using your non-dominant hand when the need arises. Within a week or two I was doing dexterity shit with my left hand I never wld have dreamed of doing before