While I haven't come across one in my studies, I would reckon the probability for one in existence is higher than 50%.
The reason being that it is a General Staff's responsibility to ponder military plans for pretty much any conceivable contingency (see my previous comment in a thread on a different topic here). Vice versa, I know for a fact that Western nuclear planning at least discussed sometime in the 1960s that a Soviet push through (neutral) Austria into Southern Germany and Northern Italy might "require" a "closing" of the "Austrian flank" by laying nuclear waste to Austria to ensure a complete halt of a Soviet advance through that territory (one can only imagine what the implied civilian casualties of such a plan were). Horrible plans are made all the time. That does not mean anyone is seriously pushing for them to be turned into action. A military planning staff's job is to provide estimates of a realizability of any necessary option in case of war. Often, such plans merely contribute to a discussion about how the outcome of such plans is not desirable. The fact that we have, in the East German/Soviet case for the invasion of West Germany, actual physical evidence for a realizable invasion (street signs and medals already printed, going way beyond a mere military contingency plan) only shows that the Warsaw pact plan for the invasion of West German and Western Europe was considered a desirable option. The fact that we do not have evidence (yet) of a concrete Warsaw pact plan to invade Switzerland the same way makes it, in my opinion, likely, that such plan was not high on the agenda, but I would be surprised if a pure military plan to invade Switzerland did not exist. I hope that makes sense.