In the 8th AF, the 20th and 55th (the first P-38 groups)were equipped with the G and H model when they went into action in late 1943, with SOME EARLY P-38J models reaching them in late 1943 and early 1944. The P-38L arrived in mid 1944, as most units in the 8th AF had already or were in the process of transitioning into the P-51.
The same basic timeline applies to the 9th AF.
New models of the P-38 were VERY slow to reach deployment. The USAAF and the War Production Board failed to adequately second source the P-38, despite the fact that they evidently felth the P-38 was so important and critical tro the war effort that NO production delays on the part of Lockheed were tolerated. The two week shutdown to switch to produce the P-38K was denied in April 1943. Yes, that's right, the P-38K should have entered service about the time the 20th and 55th went into combat with the P-38G and H.
Oddly, the ONLY plant that was chosen to second source produce the P-38 was Consolidated Vultee in Nashville Tennesse, near my home (I had family that worked there). However, Consolidated itself was a company experienced in and dedicated to production of large aircraft, where as the P-38 was builty nothing like such aircraft, and required an entirely different mode of manufacture.
Even MORE senseless was that Lockheed Burbank was tasked with building Boeing B-17 bombers, taking OVER half the plant capacity, and cutting P-38 production by more than 50%. In reality, had some of the idiots in charge gotten their heads out of their asses, the Lockheed Burbank plant could have been dedicated to production of the P-38 @ 100% capacity, and the P-38K could have been a reality. Lockheed could have produced twice as many much needed P-38's (many units flew War Wearies in combat far past their best lifetime) and brought new improved models to the front lines much faster. The last 12K of the 20K+ or so P-38's that would have resulted would have been P-38K or better models.
Consolidated Vultee would have done well building B-17s. As it was, they only produced 100 or so P-38's (113 I think). They'd have been better off making 5K or so B-17's.