Do we know that Transwarp was necessarily Autobot tech? Spacebridge technology was Decepticon tech in the G1 cartoon and developed by a neutral unaligned Transformer in the G1 comics (who became enslaved by Decepticons and thus the tech fell into their hands).
It's possible that Cybertronians were looking at improving Space Bridge technology into Transwarp Drives... thus allowing individual ships to jump through self-generated worm holes rather than having to rely on a kind of gate like the Space Bridge. The theory for transwarp drive probably existed long before it could be made into reality because often you have to wait for technology to catch up.It often happens here on Earth with science and technology, why not on Cybertron? Look at Leonardo Da Vinci... heck, the Indian scholar Pingara developed a system of binary numerals (11000010101110100111) around the 5th-2nd centuries B.C.E. -- long, long, long before the advent of modern computers today!
In the Anglophone cartoon continuity - never. In the Japanese cartoon continuity - umm... he became Super Megatron and Ultra Megatron but they both look substantially different from his original Megatron form which is what we see in the Golden Disk recording.
In the G1 comics the events of Transformers The Movie never occured in the main continuity proper, rather in alternate future continuities - which where Galvatron and Galvatron II came from. But neither of them reverted back to being Megatron (Galvatron was sent back to his original time stream and Galvatron II was buried under ice after a pretty awesome fight with Fortress Maximus). Megatron lost his original gun form in 1993 when he elicited COBRA to rebuild him as Generation 2 tank Megatron... although we're gonna see a continuity split from this later this year when IDW releases their G1 #81 which will continue from where the G1 comics ended and looks set to ignore G2.
But as it stands now, in the Marvel comics the last time we saw Megatron in his original gun form was in 1993 and the last time it was seen in the cartoon continuity was in 2005. So no later than 2005.