Upgrades EXK-1M - Upgrades an M1 to an M1 EX by adding the 4 megs of additional Multisounds that are found in the T-Series synths. EXK-M1R - Upgrades an M1R to an M1R EX by adding the 4 megs of additional Multisounds that are found in the T-Series synths. WK-T1 - For T1 only. Replaces the Joystick with Control Wheels. EXK-T - Upgrades the T2/T3 to T2 EX/T3 EX by adding 1 meg of PCM Sample RAM. Turns the T into a 1 meg sample player. The upgrade isn't for the T1 because it comes standard with this feature. InVision M1 Plus 1 - InVision made a 4 meg Multisound upgrade for the M1. Additional info can be found on the M1 Plus 1 page. ROM - Every M/T Series Synth had several ROM upgrades. They were provided free of charge from Korg back when Korg supported these synths. These were system ROM upgrades. They didn't add any new sounds just fixed problems with the synth's operating system. Do you need a ROM upgrade for your synth? If your synth is working fine then I'd say no. There are a few exceptions though... M1's with serial numbers lower than 4756 that still have the original ROM installed have problems interfacing with sound cards. If you have one of these synths there is a good chance the ROM was replaced.
T-Series Rom version 26 and up added several features such as Combi Mode Master/Multi, Copy Combination to Sequencer, and Low Memory Indicator. These features don't change the sounds. They just make using the T a little easier.
Korg doesn't make these upgrades any more but some service centers burn their own ROM chips and will do the upgrade for a fee. The Tips page has info on how to display your synth's ROM version. | 
