a practical question has just immerged 
I have a s500 quad running the great apm2.8 actually it was the first quad I ever built, still impressive though aged 5 years. I decided to build a new one using the pixhawk this time , I already buyed one and flashed it just to check if its working… I was opening the mission planner I use the mission planner 2.0 by micheal obams it gave me an update note! I updated on one of the two laptops I have but kept the other older version of mission planner 2.0 on the other and even an older mission planner 1.8 I feel I may need all, but I intend to use telemetry in this build.so I cannot get away with this two laptops thing… my question is what if I put three versions of mission planner on same laptop, any troubles may occur ???
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You only need one version of Mission Planner. The latest version. Some menu’s will not work but you can always use the Full parameter list. I was just using the latest version of Misison Planner to connect to an obsolete APM FC via telemetry.