Just another confirmation that simply renaming Insync to insync within .config (after removing/moving away the new insync folder) works here. Do this while the client is stopped, of course. (Stop the client, move away insync, rename Insync to insync, start the client).
Additional note: Even the new client 3.7.4 updated one file in the old Insync folder, namely out.txt. This made it somehow difficult to find out which one is the new one, but luckily the forums helped.
I guess automatic testing does not include existing config, but hey, everyone gets bitten once in a while, no data is really lost (not even sync config), and the reactions show (again) how nice the support is over here 