So redesign the wing along the lines of P-63. What's to be lost? 2,000 fewer P-39s? If you ended up with 1,000 better planes with longer range it might be worth it.
A lot this depends on timing. The P-39 was late to party compared to the P-40.
Production numbers for the P-38, P-39, P-40 and P-47
year........................P-38..................P-39.....................P-40.....................P-47
1940........................1..........................13.........................778.....................0
1941........................207.....................926......................2248...................1
1942........................1479..................1932..................3854...................532
1943........................2497..................4947....................4258.................4428
1944........................4186..................1729....................2002.................7065
A lot of times the bulk of production was in 2nd half of the year until 1944 for the P39 and P-40.
Back to your question, WHEN does the redesign and re tooling take place? When do accept the 1000 plane loss?
1941 and 1942 are out, you need every plane you can get. 1943 looks good but by then it doesn't matter. At some point in 1943 the P-39 becomes a Lend Lease special and domestic advance trainer.
They did end up using a lot of P-40s in training units, but the US had good advanced trainers in production like the AT-6, and overall I think a lot of the surviving older versions of fighters that were a little easier to handle like the Wildcat, P-40, Buffalo, P-36 etc. could have been used (and I think were).
AT-6 was a very good advanced flight trainer. It was not a combat trainer or advanced fighter trainer. It had a wing very close to a Hurricane in size, it weighed over 5,000lbs gross and used a 600hp engine that made max power under 10,000ft.
Max speed around 210mph and climb rate was around 1200fpm at best. Going from the A-6 to a combat fighter in a combat squadron was a recipe for disaster. Flying P-39s or P-40s in a training school for 15-30 hours (depending on year, school, instructors, etc.) meant the new pilots had a much better idea of what was going to happen when they did something,
Trying to practice highway driving in car that tops out at 40mph is not going to prepare you for highway driving at 70mph.
Please forget about the P-36 as a trainer, they were used but the US didn't have many to begin with and by 1942 a lot them were wrecked/worn out. Students were not easy on aircraft. Army was not going to use Wildcats, no spare parts and no trained mechanics. Buffalos were becoming hanger queens in the Navy. Maybe a squadron commander could keep one flying with a dedicated ground crew as a 'hack' but lack of parts and trained mechanics and their crappy landing gear was going thin them out pretty quickly.
The P-39s that were used in this role were simply pressed into it, I think, because there were so many of them and they had limited utility on the front line.
that pretty much covers it. Even near the Beginning of 1943 do front line generals want more P-39s or do they want P-47s for delivery 3-6 months in future.