At last I have contacted UG and received an answer. This here is not the original code they gave me, but my own adaption - and - big surprise - it works! It should not be a secret, it is useful for all modders, so I publish it here:
local entities = game.interface.getEntities({radius = 999999},{type = "CONSTRUCTION", includeData = false})
for _, constrId in pairs (entities) do
local constr = api.engine.getComponent(constrId, api.type.ComponentType.CONSTRUCTION)
if constr.fileName and string.find(constr.fileName, "wk_switch_kit") then -- only for my switch modules, you can also use other criteria
local newConstr = api.type.SimpleProposal.ConstructionEntity.new()
newConstr.fileName = constr.fileName
newConstr.params = game.interface.getEntity(constrId).params
for i = 1, 16 do
newConstr.transf[i] = constr.transf[i]
end
if constr.name then
newConstr.name = constr.name
else
newConstr.name = "no name"
end
newConstr.playerEntity = api.engine.util.getPlayer()
newConstr.headquarters = false
local proposal = api.type.SimpleProposal.new()
proposal.old2new = {[constrId] = 0} -- 0! Not -1! What a surprise! :-)
proposal.constructionsToRemove = {constrId}
proposal.constructionsToAdd[1] = newConstr
api.cmd.sendCommand(api.cmd.make.buildProposal(proposal, nil, false), function(res, success)
--- insert individual error handler
end)
end
endThere were some mistakes in my own code. But the decisive one was proposal.old2new. The proposal entities start at 0! Who could know this?! And also the syntax is different from what UG has written in the wiki. And I had to guess where to get the params from: There are several possibilities, but some of them are wrong. I think the official documentation is a little poor. 
But just having finished this, I face a new problem: If you have got constructions with edges and nodes, the nodes will be cut. There is no more connection with the neighboring edges. This happens only with constructions with rigid edges, not with normal edges. So I'm looking for the cause, but I have no idea. Remove and insert the edges without their constructions is no good idea - it will crash the mod.
Now I am searching for a structure which describes the connections between neighboring edges and a mechanism to reconnect them. Any idea?
If you ask me why I need so difficult things: My project is to replace objects by themselves to update e.g. track styles. If you install a new track/street or track/street modifying mod, you normally don't have any chance to update already laid tracks/streets. So you have to replace them manually. 
I want to create a mod which does this work for me. 
But you also can imagine other applications. You could create tools to cut - copy - paste streets, tracks end even whole cities - or repair maps which you have created with no more available mods.
), but construction proposals seem to be a different thing. No modder did use this before, not even you. 
(oder ist es Fachchinesisch?):
usw.
Danke! Update war der richtige Weg! Aber du hattest es auch etwas anders strukturiert, als ich es in einem anderen Beispiel vorfand. Keine Ahnung, was da falsch war. Allerdings wird es mit Update permanent ausgegeben. Um die Daten einmalig abzufragen, bleibt wohl nichts anderes übrig, als ein Flag zu setzen. 
